Optical tuning of light emitting semiconductor junctions

ABSTRACT

Light emitting semiconductor junctions are disclosed. An exemplary light emitting junction has a first electrical contact coupled to a first side of the junction. The exemplary junction also has a second electrical contact coupled to a second side of the junction. The exemplary junction also has a region of set straining material that exerts a strain on the junction and alters both: (i) an optical polarization, and (ii) an emission wavelength of the junction. The region of set straining material is not on a current path between said first electrical contact and said second electrical contact. The region of set straining material covers a third side and a fourth side of the light emitting junction along a cross section of the light emitting junction. The light emitting semiconductor junction device comprises a three-five alloy.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.61/844,228, filed Jul. 9, 2013, which is incorporated by referenceherein in its entirety.

BACKGROUND OF THE INVENTION

Semiconductor optoelectronic devices are efficient energy transformerswhich can convert electrical energy into optical energy. The reciprocalprocess is also possible wherein optical energy is converted directlyinto electrical energy. Optically emissive devices that generate lightfrom an electrical source are used as solid-state semiconductor lightsources such as incoherent light emitting diodes (LEDs) and coherentlight emitting laser diodes. Semiconductor light sources take advantageof the interaction of optical energy with the semiconductor's crystalstructure which has a specific electronic energy configuration known asthe electronic band structure.

The industrial and commercial potential for LEDs and semiconductorlasers continues to expand. Although LEDs have been available since the1960s, the efficiency of LEDs has continuously improved since that time,and solid-state lighting applications using LEDs are now considered aviable alternative to incandescent and high intensity discharge lightsources. Modern LEDs produce light across the visible and near infraredelectromagnetic spectrum, and researchers have developed semiconductormaterials for LEDs that produce deep ultraviolet (DUV) light typicallyoperating in the optical wavelength range of 190 nm to 290 nm. DUV-LEDsproducing incoherent light have significant commercial potential forsurface and air sterilization or sanitation, extremely high-densityoptical storage having high speed read & write ability that iscompetitive to nanoscale imprint technologies, fine geometrylithography, and information processing systems generally. Inparticular, optical communication links using the deep ultraviolet bandoffers unique short range device connectivity. Various additionalapplications that require efficient and compact sources for ultravioletlight are enabled by DUV-LEDs such as those involving the interrogationof biological systems.

Semiconductor light sources generate light using semiconductor junctionscomprising at least a p-type semiconductor region and an n-typesemiconductor region. The p-type semiconductor region is designed to bea source of holes, whereas the n-type region is a source of electrons.Under the appropriate external electrical bias, electron and holes areinjected from their respective sources towards anelectron-hole-recombination region (EHR)—which can be described withreference to FIG. 1 a.

For the case of a p-n junction diode, the electron-hole-recombinationregion is substantially the depletion region between the p-type andn-type semiconductors. Alternatively, an intentional intrinsic region ornot-intentionally doped (NID) region can be inserted between the p-typeand n-type regions to form a p-i-n diode. A p-i-n diode is designed suchthat a majority of the electron-hole-recombination occurs spatiallywithin the intrinsic or NID region.

FIG. 1a illustrates a semiconductor junction 100 comprising distinctspatial regions of semiconductor material types and the associatedenergy band diagram 101 along a spatial direction comprising theordinate of the diagram. The abscissa of energy band diagram 101correlates spatially with cross section 102 of semiconductor junction100. In other words, the left side of energy band diagram 101corresponds to the p-type portion 103 of semiconductor junction 100, andthe right side of energy band diagram 101 corresponds to the n-typeportion 104 of semiconductor junction 100. The p-type portion 103 of thesemiconductor junction comprises mobile positive charge carriers in theform of holes 105. The n-type portion 104 of the semiconductor junctioncomprises mobile negative charge carriers in the form of electrons 106.The electrons and holes can diffuse and drift toward the depletionregion or intrinsic region 107. For the case of p-n and p-i-n junctions,the regions 103, 107 and 104 can comprise a uniform bandgap compositionand can collectively be referred to as a homojunction. If anyone or allof the regions 103, 107 or 104 comprise dissimilar semiconductorcompositions of differing bandgap, then a so called heterojunctiondevice is formed. For the case of a p-n device, there will be anequilibrium reaction wherein electrons and holes diffuse across theinterface defining a p-type region and an n-type region forming abuilt-in potential. This built-in potential establishes a depletionregion that is neither n-type nor p-type and is ideal for functioning asan EHR. In p-i-n junctions, the region 107 is additionally andintentionally engineered with a predetermined dimension which is oftenlarger than the depletion region formed by an abrupt p-n junction. Thisis particularly useful for controlling the spatial extent of EHR for thepurpose of photon generation and extraction of said photon externallyfrom the device.

The mechanism by which semiconductor junction 100 generates lightcomprising photons having distinct energy spectrum (or equivalentwavelength spectrum) is by the simultaneous particle recombination ofspatially coincident electrons in the lowest energy portion of theconduction band with holes in the highest portion of the valence band.That is, the process of generating a photon of energy E_(p) isdetermined by the energy difference between an electron in theconduction band and hole in the valence band where E_(p)=E_(c)(k=0,z)−E_(v)(k=0, z)˜E_(G). The respective electron and hole energies are ingeneral taken at the same semiconductor crystal momentum vector k of thehost semiconductor band structure (having a distinct energy-momentumdispersion defining said band structure) in order to conserve energy andmomentum during the conversion process. This is due to the fact that aphoton has zero momentum. High efficiency optical generation viaelectron-hole recombination process occurs for so-called direct bandgapsemiconductors wherein the energy-momentum dispersion has the lowestlying energy of conduction band occurring at k=0 and the highest lyingvalence band also occurring at k=0. In general, all optical propertiesof interest for light emission occur in the vicinity of the so calledzone center of the band structure, which is centered at k=0. Therefore,the bandgap of the host semiconductor represented by E_(G)≡E_(G)(k=0) istypically defined as the energy difference between said lowest energyconduction and highest valence band energies at k=0. This allowsrepresentations of the spatial band structure of p-n and p-i-n devicesto be abbreviated by their k=0 representation, as described withreference to FIG. 1a . Note, in general E_(p)≦E_(G) due to the coulombattraction of the electron and hole forming an intermediary particlecalled an exciton having binding energy E_(B).

The energy band diagram 101 of the layered p-i-n diode 100 of FIG. 1a istherefore representative of a homogeneous bandgap semiconductorspatially modified along a direction comprising the ordinate of thediagram, so as to exhibit a distinct p-type 103, an optional NID ordepletion region 107 and an n-type region 104. The so called conductionband edge E_(c)(k=0, z) 108 represents the k=0 energy position of thelowest energy conduction band of the host semiconductor, whereas thevalence band edge E_(v)(k=0, z) 109 represents the highest valence bandenergy position relative to the conduction band edge 108.

The p-type 103 and n-type 104 regions can be electrically contacted byideal low resistance ohmic contacts forming a two electrical terminaldevice. Under appropriate externally applied electrical bias applied tothe contacts, the junction of FIG. 1a can be operated to function inforward bias mode wherein mobile electrons 106 in region 104 and holes105 in region 103 are injected toward region 107. For the case of a p-nhomojunction, region 107 is the well known depletion region of width WDsetup by the abrupt junction formed between the n-type 104 and p-typeregion 103. The built-in electric field along the device direction,comprising the ordinate of the diagram, is determined by the differencein acceptor and donor doping concentrations in regions 103 and 104,respectively, and the applied electrical bias.

For the case of a p-i-n diode, the intrinsic region has a width that isintentionally engineered to have width W_(i) and is generally notintentionally doped with impurity species or is advantageouslycompensated or chemically modified so that the Fermi energy E_(F) lieswholly within the bandgap of the host semiconductor. Preferably, theintrinsic region should be devoid of non-radiative traps or loss pathsprovided by structural defects and have E_(F) positioned approximatelyto half of E_(G).

Once the light has been generated in the semiconductor junction it willneed to be coupled out from the interior of the device and emittedexternally for utility as a light source. In this regard, the design ofsemiconductor laser diodes and LEDs differ significantly. Both LEDs andlaser diodes can be designed to be of a vertically emitting type (i.e.,light is generated within the device and emitted substantiallyperpendicular to the plane of the layers) or a waveguide type (i.e.,light generated and guided within the device and emitted substantiallyparallel to the plane of the layers). In both of these optical designconfigurations, the optical polarization of the generated light is ofcritical importance.

In general, lasers require the additional feature of parallel reflectivelayers bounding the optical generation region in order to recyclephotons back into the EHR region for the express purpose of producingstimulated emission and thus coherent light. In contrast, LEDs aregenerally classed as producing photons due to spontaneous emissionprocess and thus produce incoherent light output. LEDs can produce lightthat is optically polarized or non-polarized, whereas laser diodesproduce polarized emitted light. The polarization state of a givendevice is determined by both the semiconductor bandstructure and theoptical structure configuration, namely, vertical emitter or waveguidetype. The choice of any one particular optical configuration usingspecific semiconductor materials will be determined by the opticalproperties of the light required for a given application and the cost ofproducing the device. In general, vertically emissive devices are oflower cost than waveguide type devices. However, achieving high opticalextraction efficiency (which is the ratio of the amount of lightgenerated within the device to the amount of light extracted externallyfrom the device), is often the prime goal. This has proven to be alimiting attribute for deep ultraviolet LEDs wherein the extractionefficiency is fundamentally limited by the semiconductor band structureand the high optical refractive index of the semiconductor materialitself.

The effect of the optical polarization of light internally generated bya semiconductor junction on the performance of the device can bedescribed with reference to FIG. 1b . This figure displays asemiconductor structure 110 comprising an active layer 111 and asubstrate 112. Substrate 112 is generally a single crystal material ifused for subsequent epitaxial deposition of active layers 111.Alternatively, substrate 112 may be non-crystalline or amorphous if usedas a mechanical support or as an optical coupler for otherwisemechanically transferred crystalline active layers 111.

Substrate 112 may further be either selected from an optically opaque ortransparent material if utilized as a crystalline template for seedingepitaxial layers 111. If the substrate is transparent to the designemission wavelength, then light can be coupled externally from thedevice layers 111. If the crystalline substrate 112 is opaque to thedesign wavelength, then optical energy can be coupled out from thedevice laterally as a waveguide or from the topmost surface. Yet afurther method of coupling energy out from active layers 111 using anopaque substrate is to remove a portion of the substrate materialbeneath the active region of a device so as to enable emission throughan aperture in the substrate.

For application of homojunction and heterojunction LEDs to deepultraviolet wavelengths in the range of 200 to 260 nm, the wide band gapsemiconductor materials having a composition ofaluminium-gallium-indium-nitride (with chemical formulaAl_(x)Ga_(y)In_(1-x-y)N, where 0≦x≦1, 0≦y≦1) and magnesium-zinc-oxide(Mg_(x)Zn_(1-x)O where 0≦x≦1, 0≦y≦1) are of particular interest. Othermaterial compositions are also possible. However, the technologicallymature AlGaInN and MgZnO materials have been shown to be able to formstable and continuous compositions of ternary and quaternary alloys byvarying the mole fractions of the incorporated metals within the nitrideor oxide crystalline structure.

The group-III Nitride material is presently the most mature wide bandgap semiconductor material and is widely used in near ultraviolet andvisible LEDs in the wavelength range of 250 to 600 nm. Unfortunately,deep ultraviolet LEDs based on AlGaN, AlInGaN or AlInN operating withbandgap energies in the range from about 200 nm to approximately 260 nmsuffers a deleterious and fundamental limitation of the electronic bandstructure that is inefficient for vertical type emitters.

Furthermore, heterojunction LED devices using two or more dissimilarcomposition of group-III Nitrides formed as a wurtzite crystal structuresuffer from the disadvantageous creation of internal spontaneous andpiezoelectric electric fields due to extremely large inducedpolarization charges at each heterojunction interface. That is, at eachinterface (for example the interface betweenAl_(x)Ga_(1-x)N/Al_(y)Ga_(1-y)N where x≠y) these polarization chargesgenerate built-in electric polarization fields which tend to prevent theefficient spatial localization of electrons and holes, and thus exhibitpoor electrical-to-optical generation efficiency.

The internal polarization charges at a group-III Nitride heterojunctioninterface can be reduced and potentially eliminated if grown epitaxiallyas substantially single crystal films on semi-polar or non-polarsurfaces, and have been identified in the prior art. Unfortunately,epitaxial deposition of thin films of wurtzite group-III Nitrides onnon-polar crystal surfaces typically result in poor crystal qualityfilms when compared to a deposition on the so called polar surfacec-plane having hexagonal unit cell crystal symmetry. Thus the higheststructural quality material for group-III Nitrides (and indeed also forMgZnO) is obtained by depositing on the polar c-plane and in turnresults in the highest polar type of crystal with largest internalpolarization charge for a given heterojunction interface. Thin filmsingle crystal group-III Nitrides deposited along a growth directionwhich is substantially perpendicular to the c-plane therefore results inthe two distinct crystal structure film types labelled as metal-polarand nitrogen-polar. Achieving a group-III Nitride structure thatexhibits pure polarity-type, namely metal-polar and nitrogen polar is ingeneral a desirable goal for device operation. Introduction ofstructural defects increases the probability of creating mixed polaritydomains within the films during epitaxy.

There is yet a further problem with prior art group-III Nitride bulksubstrate growth and epitaxial deposition methods. Native single crystalgroup-III Nitride binary substrates of GaN and AlN are available withlow defect density but have relatively high cost of manufacture whencompared to high quality single crystal sapphire and silicon. Epitaxialgrowth of electronic and optical devices using single crystal low defectdensity ternary AlGaN and AlInAl or quaternary AlInGaN films are alsolimited by the in-plane lattice constant mismatch Δa of the film withrespect to the crystalline substrate. Therefore, Δa is a function ofgroup-III Nitride epitaxial material composition relative to theunderlying substrate lattice constant.

In general, epitaxial layer deposition degrades in structural qualityfor a given group-III Nitride composition when the individual layerthickness exceeds the so called critical layer thickness (CLT) whendeposited on a dissimilar substrate. The critical layer thickness isdefined as the maximum epitaxial layer thickness that can accommodatethe in-plane lattice distortion of the crystal unit cell elasticallywithout creating misfit dislocations. The CLT correlates strongly withthe lattice mismatch. This places severe limitations on the materialcombinations of epitaxially layered heterojunction devices if structuraldefects are required to be low.

In particular, for the case of LEDs and laser diodes, these structuralcrystal defects arising from lattice constant mismatch of dissimilargroup-III Nitride films present alternative electron and holerecombination pathways that are non-radiative in nature and thusrepresent a major production loss for the desired emission energyengineered by the bandgap or heterojunction device.

Therefore, the lattice constant mismatch problem of heterojunctions andmultilayered stacks of dissimilar composition films (each having anassociated distinct band structure and crystal lattice constant) limitsthe ultimate internal quantum efficiency and design range of emissionwavelengths for optically emissive devices. The management of internalelastic strain of each of the dissimilar layers comprising aheterojunction device or multilayered stack comprising a plurality ofdissimilar composition films is important for controlling the finalcrystal structure quality of the device. In practice, the previouslydiscussed lattice mismatch issues place severe limitation on the choicecompositions selected for constructing a given device and thus thedesign space of possible bandgap engineered devices.

Yet a further major problem in the group-III Nitrides is the depositionprocess complexity for creating multiple layers of dissimilarcompositions with a wide range of alloy compositions within a givendeposition process, albeit by using either chemical vapor deposition(CVD), metal organic CVD (MOCVD), molecular beam epitaxy (MBE), vaporphase epitaxy (VPE), sputter deposition, ion beam deposition, and othermethods. That is, in general large variation in the composition of thegroup-III Nitrides alloys typically require distinct deposition processspecification to achieve high structural quality films. For example,Al-rich films of a given Al_(x)Ga1-xN alloy (x>0.5) require differentprocess conditions compared to Ga-rich films (x<0.5) such as differentconstituent metal-to-active nitrogen ratios during deposition anddifferent growth temperatures. Therefore, high quality multilayered thinfilm epitaxial structures comprising a plurality of widely varying AlGaNfilm compositions require a complex growth process and thus have anincreased cost and potentially lower final structure production yielddue to normal deposition process variability.

SUMMARY OF INVENTION

In one embodiment, a new method for tuning the electronic and opticalproperty of a semiconductor used for an optically emissive device toachieve a predetermined optical state is provided. The desired opticalstate is specified as the emission wavelength and optical polarization.The method tunes the final optical and electrical properties of anoptically emissive device using a post semiconductor layer formationprocess and is during the physical device formation steps. The methodallows for the tuning of both the optical emission wavelength and theoptical polarization emitted by the device using a post semiconductordeposition band structure tuning process.

In another embodiment, a new method to circumvent the limitations in theprior art for application to deep ultraviolet optically emissive deviceoperating in the wavelength range of 190-250 nm using wide band gapsemiconductors is provided. The wide band gap semiconductors can includecompositions of group-III Nitrides such as binary AlN, binary GaN andbinary InN and or ternary AlGaN, AlnN, and or quaternary AlGaInN. Thewide band gap semiconductors can also include compositions of group-IImetal Oxides such as MgZnO. The wide band gap semiconductors can alsoinclude compositions of boron-aluminium-nitride (B_(x)Al_(1-x)N).

In another embodiment, a post epitaxial semiconductor formation processis used to engineer a stressor region on at least a portion of the saidsemiconductor region to improve the local band structure characteristicsdefined by a region substantially bounded by said stressor. At least oneof a desired optical property or a desired activated dopantconcentration is achieved by introducing the post epitaxial stressorformed as described herein.

In another embodiment, a semiconductor homojunction or heterojunction isinitially formed via a semiconductor formation process. Thesemiconductor is then formed into an optically emissive devicecomprising a plurality of physically subtractive and additive processesto achieve formation of electrical contacts and optical coupling layers.During the device formation process but after the epitaxial orsemiconductor film formation, externally applied stressors areengineered advantageously to achieve a desired optical state duringcompleted device operation.

In another embodiment, an ultraviolet light emitting diode is provided.The ultraviolet light emitting diode comprises an ultraviolet lightemitting junction. The diode also comprises a first electrical contactcoupled to a first side of the junction. The diode also comprises asecond electrical contact coupled to a second side of the junction. Thediode also comprises a region of set straining material that exerts astrain on the junction and alters both: (i) an optical polarization, and(ii) an emission wavelength of the junction. The region of set strainingmaterial lies outside a current path that includes the first electricalcontact and the second electrical contact. The light emittingsemiconductor junction device comprises a three-five alloy.

In another embodiment, a light emitting semiconductor junction device isprovided. The light emitting semiconductor junction comprises a lightemitting junction. The light emitting semiconductor junction alsocomprises a first electrical contact coupled to a first side of thejunction. The light emitting semiconductor junction also comprises asecond electrical contact coupled to a second side of the junction. Thelight emitting semiconductor junction also comprises a region of setstraining material that exerts a strain on the junction and alters both:(i) an optical polarization, and (ii) an emission wavelength of thejunction. The first electrical contact and the second electrical contactprovide a current path to the light emitting junction independent of theregion of set straining material. The region of set straining materialcovers a third side and a fourth side of the light emitting junctionalong a cross section of the light emitting junction. The light emittingsemiconductor junction device comprises a three-five alloy.

In another embodiment, a light emitting semiconductor junction devicecomprises a light emitting junction. The light emitting semiconductorjunction device also comprises a first electrical contact coupled to afirst side of the junction. The light emitting semiconductor junctiondevice also comprises a second electrical contact coupled to a secondside of the junction. The light emitting semiconductor junction devicealso comprises a region of set straining material that exerts a strainon the junction and alters both: (i) an optical polarization, and (ii)an emission wavelength of the junction. The region of set strainingmaterial is not on a current path between said first electrical contactand said second electrical contact. The region of set straining materialcovers a third side and a fourth side of the light emitting junctionalong a cross section of the light emitting junction. The light emittingsemiconductor junction device comprises a three-five alloy.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A illustrates a light emitting semiconductor structure that isfound in the related art. The recombination of electrons and holeswithin a p-i-n diode is described.

FIG. 1B illustrates a light emitting semiconductor structure that isfound in the related art. The escape cone and total internal reflectionof light generated within the interior of the device is described.

FIG. 2 illustrates a light emitting semiconductor structure that isfound in the related art. The equivalent electrical circuit and basicoptical function of a light emitting device is described.

FIG. 3A illustrates an atomic crystal semiconductor structure that isfound in the related art. An ideal unit cell of a wurtzite semiconductorcrystal is described.

FIG. 3B illustrates a light emitting semiconductor structure that isfound in the related art. The equivalent representation of the hexagonalwurtzite unit cell is shown.

FIG. 4 illustrates a wurtzite c-plane atomic arrangement of a wurtzitesemiconductor structure that is found in the related art.

FIG. 5 illustrates a 3D wurtzite atomic arrangement of a wurtzitesemiconductor structure. A calculated finite 3D slab of orientedwurtzite material is disclosed. The slab is shown oriented along a highsymmetry crystal growth direction.

FIG. 6 illustrates an oriented 3D wurtzite crystal slab with atomicarrangement of Nitrogen-polar bonds along the growth axis. Theelectrical doped layer regions form an oriented wurtzite vertical p-i-ndiode.

FIG. 7 illustrates an oriented 3D wurtzite crystal slab with atomicarrangement of Nitrogen-polar bonds along the growth axis. Theelectrical doped regions form an oriented wurtzite lateral p-i-n diode.

FIG. 8 illustrates a representation of an oriented wurtzite verticalp-i-n structure disposed upon a substrate.

FIG. 9 illustrates a representation of an oriented wurtzite lateralp-i-n structure disposed upon a substrate.

FIG. 10 illustrates the calculated spatial band structure of a group-IIINitride p-n junction.

FIG. 11 illustrates the calculated spatial band structure of a group-IIINitride p-i-n junction.

FIG. 12A illustrates a portion of the atomic arrangement of a wurtzitecrystal found in an oriented plane. The oriented plane represents theidealized c-plane of a wurtzite crystal.

FIG. 12B illustrates a portion of the atomic arrangement of a wurtzitecrystal found in an oriented plane taken along the crystal growth axis.The diagram illustrates ideal epitaxial growth of dissimilar latticeconstant wurtzite materials undergoing elastic deformation.

FIG. 13A illustrates an oriented wurztite film that is used as thestarting surface for lattice mismatched epitaxy by a larger latticeconstant wurtzite surface. The atomic arrangement is depictedspecifically in the c-plane.

FIG. 13B illustrates tensile bi-axial in-plane forces upon an orientedwurztite film during lattice mismatched epitaxy on a larger latticeconstant wurtzite surface. The atomic arrangement is depictedspecifically in the c-plane.

FIG. 14A illustrates the c-plane atomic arrangement a wurztitesemiconductor. A metal terminated surface is shown.

FIG. 14B illustrates the c-plane oriented wurtzite structure of FIG. 14Aunder the influence of a uni-axial stressor.

FIG. 15 illustrates the orientation of a uniaxial stressor applied to aspecific crystal direction of a 3D wurtzite crystal slab. The uniaxialstressor is applied perpendicular to the growth axis.

FIG. 16A illustrates a representation of a strain-free energy-momentumband diagram of a wurtzite semiconductor. The band diagram furtherrepresents an example high Al % group-III Nitride compound with valenceband ordering as shown.

FIG. 16B illustrates a strained energy-momentum band diagram of awurtzite semiconductor. The band diagram further represents an examplehigh Al % group-III Nitride compound with valence band ordering as shownsubjected to a large in-plane compressive stress.

FIG. 17A illustrates a representation of a strain-free energy-momentumband diagram of a wurtzite semiconductor. The band diagram furtherrepresents an example group-III Nitride compound with valence bandordering as shown.

FIG. 17B illustrates a strained energy-momentum band diagram of awurtzite semiconductor. The band diagram further represents an examplegroup-III Nitride compound with valence band ordering as shown subjectedto a large in-plane tensile stress.

FIG. 18 illustrates the calculated valence band energy and orderingvariation under various in-plane strain conditions for the case of ahigh Al % group-III Nitride wurtzite semiconductor. The opticalpolarization property of the lowest energy valence band is indicated asTE or TM.

FIG. 19 illustrates the calculated energy band structure comprising theconduction band and the three high symmetry valence bands of a wurtzitesemiconductor under the influence of various in-plane strains.

FIG. 20 illustrates the variation in the fundamental energy band gap ofthree representative group-III Nitride compounds under the condition ofin-plane strain. The transition from optical polarizations of TM to TEis indicated.

FIG. 21A illustrates the spatial energy band structure of a high Al %wurtzite AlGaN homojunction p-i-n diode subjected to a large in-planecompressive uniaxial stressor.

FIG. 21B illustrates the 3D representation a homojunction p-i-n diodesubjected to a large in-plane compressive uniaxial stressor orientedwith respect to a crystal axis.

FIG. 22A illustrates the spatial energy band structure of a high Al %wurtzite AlGaN homojunction p-i-n diode subjected to a large in-planetensile uniaxial stressor.

FIG. 22B illustrates the 3D representation a homojunction p-i-n diodesubjected to a large in-plane tensile uniaxial stressor oriented withrespect to a crystal axis.

FIG. 23A illustrates a 3D implementation a cylindrical symmetry p-i-ndiode comprising a wurtzite semiconductor. The cutaway view shows theinterior of the device and position of the cylindrical stressor.

FIG. 23B illustrates a 2D cross-section of implementation a cylindricalsymmetry p-i-n diode comprising a wurtzite semiconductor. Thecross-sectional view shows the interior of the device and position ofthe cylindrical stressor.

FIG. 23C illustrates a 2D cross-section of implementation a cylindricalsymmetry p-i-n diode comprising a wurtzite semiconductor. Thecross-sectional view shows the interior of the device and position ofthe cylindrical stressor.

FIG. 24 illustrates a 3D implementation a cylindrical symmetry p-i-ndiode comprising a wurtzite semiconductor. The device shows the positionof the cylindrical stressor and the in-plane radial symmetriccompressive stress applied to the p-i-n diode due to the stressor.

FIG. 25 illustrates a 3D implementation a cylindrical symmetry p-i-ndiode comprising a wurtzite semiconductor. The device shows the positionof the cylindrical stressor and the in-plane radial symmetric tensilestress applied to the p-i-n diode due to the stressor.

FIG. 26A illustrates a vertical type p-i-n homojunction device withelectrical contacts formed into a hexagonal symmetry column.

FIG. 26B illustrates the vertical type p-i-n homojunction device withelectrical contacts formed into a hexagonal symmetry column of FIG. 26Awith a further in-plane stressor coating applied to the mesa sidewall.

FIG. 27 illustrates a top view of an integrated light emitting devicecomprising a plurality of hexagonal p-i-n mesa diodes close-packed andelectrically interconnected. Each p-i-n mesa has an in-plane stressor asshown in FIG. 26B.

FIG. 28 illustrates a 3D view of the integrated light emitting devicecomprising a plurality of hexagonal p-i-n mesa diodes close-packed andelectrically interconnected. Each p-i-n mesa has an in-plane stressor asshown in FIG. 26B.

FIG. 29 illustrates an example of a plurality of lateral uniaxialstressors applied to a plurality of p-i-n mesa regions disposed across aplanar substrate.

FIG. 30 illustrates an example of a plurality of lateral uniaxialstressors applied to a plurality of p-i-n mesa regions disposed across aplanar substrate. The uniaxial compressive in-plane stressor action isshown explicitly acting on the rectangular p-i-n mesa regions.

FIG. 31 illustrates an example of a plurality of lateral uniaxialstressors applied to a plurality of p-i-n mesa regions disposed across aplanar substrate with the electrical bus bars interconnecting theplurality of p-i-n diodes. The stressor regions incorporate anelectrical connection to a lower conductivity type region of the p-i-ndiodes. The electrical contacts are show to co-exist in the same upperplane.

FIG. 32 illustrates a schematic representation of plurality of in-planestressors applied to group-III nitride semiconductor. The stressors canbe wholly within a plane of the layer or disposed along a growthdirection of the semiconductor.

FIG. 33 illustrates the 2D model of a straight side walled wurtziteAlGaN mesa disposed upon a substrate and capped with a passivation layerof Al₂O₃. The lateral stressors impart a compressive in-plane straininto the AlGaN mesa. The AlGaN is oriented upon a c-plane according tothe present invention.

FIG. 34 illustrates the 2D spatial variation of the stress tensorexx(x,z) in a region containing the strained AlGaN mesa.

FIG. 35 illustrates the 2D spatial variation of the stress tensorexz(x,z) in a region containing the strained AlGaN mesa.

FIG. 36 illustrates the 2D spatial variation of the stress tensorezz(x,z) in a region containing the strained AlGaN mesa.

FIG. 37 illustrates the 2D spatial variation of the conduction bandenergy E_(C)(x,z) in a region containing the strained AlGaN mesa.

FIG. 38 illustrates the 2D spatial variation of the crystal split bandenergy E_(CH)(x,z) in a region containing the strained AlGaN mesa.

FIG. 39 illustrates the 2D spatial variation of the heavy-hole bandenergy E_(HH)(x,z) in a region containing the strained AlGaN mesa.

FIG. 40 illustrates the 2D spatial variation of the light-hole bandenergy E_(LH)(x,z) in a region containing the strained AlGaN mesa.

FIG. 41 illustrates the 2D spatial variation of the lowest transitionenergy between conduction band to light-hole band in a region containingthe strained AlGaN mesa.

FIG. 42 illustrates the 2D model of a tapered side walled wurtzite AlGaNmesa disposed upon a substrate and capped with a passivation layer ofAl₂O₃. The lateral stressors impart a compressive in-plane strain intothe AlGaN mesa. The AlGaN is oriented upon a c-plane.

FIG. 43 illustrates the 2D spatial variation of the stress tensorexx(x,z) in a region containing the strained AlGaN mesa.

FIG. 44 illustrates the 2D spatial variation of the stress tensorexz(x,z) in a region containing the strained AlGaN mesa.

FIG. 45 illustrates the 2D spatial variation of the stress tensorezz(x,z) in a region containing the strained AlGaN mesa.

FIG. 46 illustrates the 2D spatial variation of the conduction bandenergy E_(C)(x,z) in a region containing the strained AlGaN mesa.

FIG. 47 illustrates the 2D spatial variation of the crystal split bandenergy E_(HH)(x,z) in a region containing the strained AlGaN mesa.

FIG. 48 illustrates a 3D representation of p-i-n device subjected to twotypes of stressors. A first stressor is disposed on the topmost surfaceof the device. A second stressor is created by etching a trench in thesemiconductor. An optional undercut etch beneath a portion of the activedevice is shown.

FIGS. 49A and 49B illustrate cross sections of various strain inducingtrench processes.

FIGS. 50A and 50B illustrate cross sections of a strain tuned lightgenerating semiconductor junction devices using a strain inducingsheathing layer.

FIG. 51 shows a comparison of two example stressor types provided by asheathing stressor and a cylindrical stressor applied to a wurtziteactive region.

FIG. 52 illustrate a process for producing a strain tuned lightgenerating semiconductor junction device using a back side straininducing layer.

FIG. 53 illustrate a process for producing a strain tuned lightgenerating semiconductor junction device using ion implantation anddiffusion.

FIG. 54 illustrates a process of fabricating a DUV-LED with a desiredemission wavelength.

FIG. 55 illustrates a process for calibrating a fabrication line forproducing devices with light emitting semiconductor junctions.

FIG. 56 illustrates a process for fabricating a light emittingsemiconductor junction.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Desired improvements over the conventional application of group-IIInitride semiconductors to the epitaxial fabrication of LED andlaser-diodes include: (i) lowering the production cost of the device;(ii) increasing the thin film formation and device yield to a targetspecification; (iii) increasing the crystalline epitaxial structurequality (i.e., having lower defect density); (iv) reducing problematicinternal electric polarization fields generated in heterojunctiondevices; (v) increasing the extraction efficiency of light generatedwithin the optical device; (vi) increasing the lifetime and reliabilityof device; and (vii) reducing dependence on crystal polarity variation.

The following disclosure includes a description of light emittingsemiconductor junction devices that are optically tuned using anengineered strain inducing region that is formed across at least aportion of the device. The strain inducing region can be formed afterthe light emitting junction itself has been formed and made operable togenerate light in response to an electronic stimulus. For example, thestrain inducing region could be formed after the active semiconductorlayers have been deposited upon a substrate and also after theelectrical contacts have been formed for the light emitting junction. Inyet another example, the strain inducing region could be formed afterthe active semiconductor layers have been deposited upon a substrate andintegrated as part of the formation steps defining the electricalcontacts.

The characteristics of light generated by a light emitting junction maybe measured before the strain inducing region is formed. Thesecharacteristics could include the wavelength and the opticalpolarization of the light generated by the active semiconductor layers.This can readily be performed using a whole wafer mapping method such anon-contact or non-destructive characterization technique. For example,photoluminescence readily provides spatial and spectral mapping ofsemiconductor properties across an entire wafer area. Once thesemiconductor properties are established, then a tailored set straininducing region is engineered in order to alter the wavelength andpolarity of the desired generated light. The strain inducing region isthen initially set during the device fabrication process. The benefit ofa set strain inducing region is that it can be formed using global waferprocessing steps, and can therefore alter the characteristics of a largenumber of devices at the same time.

In general, non-uniformity in semiconductor material properties across awafer area occurs in two forms. A first non-uniformity occurs on theatomistic scale with a length scale of angstroms to tens of nanometers.A second non-uniformity occurs on the length scale of millimeters tocentimeters and has a gradual variation across the wafer surface. Thissecond type of non-uniformity is of particular interest to postepitaxial semiconductor layer deposition, and results from temperatureand deposition specie non-uniformities. An example of these larger scalegradual variations (such as ternary band gap energy) may result inradial variations out from the center of the wafer or a transition in aproperty from one side of a wafer to an opposite side (these trends arecharacteristic of particular deposition processes, namely, single waferor multi-wafer processes). As such, a post epitaxial depositioncorrective method for the non-uniformity is valuable for improving thewhole on wafer device yield in view of the epitaxial film orsemiconductor non-uniformity. The spatially engineered strain inducingregions can therefore be tailored to specific portions of thecharacterized semiconductor wafer having non-uniform property. Thestrain inducing step can be method of further additive or subtractivematerial or via implantation technique of foreign species.

Conversely, the post epitaxial strain inducing method provides anopportunity for attaining a desired optical and electronic property of asemiconductor that is not able to be formed by epitaxy alone. Forexample, high aluminium percent AlGaN semiconductor alloys used for deepultraviolet device application require bandgap energies of 6.195 eV (200nm) to 4.765 eV (260 nm). At these AlGaN compositions the electronicband structure produces optical emission that does not favor verticallyemissive device operation but rather waveguide type. This is due to thevalence band energy and symmetry type inherent to said alloy whichdetermines the optical polarization state of the lowest energy bandgap—and therefore dominates the optical properties. Therefore, to createvertically emissive devices from the aforementioned high aluminiumpercent (Al %) AlGaN alloy, it is possible to impose a post growthstrain inducing region to alter the electronic properties of thesemiconductor alloy band structure so that it does become responsive tovertically emissive behaviour. That is, post semiconductor growth straininducing method can be used to tune the electronic band structure ofsaid semiconductor.

Yet a further opportunity for post semiconductor deposition modificationof the electronic band structure is for tuning the desired lowest energybandgap and thus the optical emission energy of the final opticallyemissive device. In particular, group-III Nitrides are extremelysusceptible to strain induced modification of the electronic bandstructure. Even more so are the high Al % AlGaN alloys, which are ofparticular interest to the examples described herein.

Yet a further aspect of the examples described herein is the ability tostandardize the epitaxial semiconductor wafer structure so as toincrease the uniformity (and thus yield) of the semiconductor propertiesfor a given semiconductor formation process. This has the benefit offurther reducing the active epitaxial or semiconductor wafer cost. Thenusing strain induced tuning of said active semiconductor via wafer scaleprocessing, many different optically emissive wavelength and opticalpolarization type devices can be fabricated using a standardized wafercomposition or structure. For example, a bulk-like AlGaN alloy wafer canbe formed as a p-i-n homojunction epitaxial structure. Thick filmhomogeneous composition AlGaN can be deposited to achieve extremely lowlevels of disadvantageous structural defects. This is due to reducingthe limitations of lattice constant mismatch of dissimilar compositionAlGaN materials. Then the formed active semiconductor layers comprisingthe delivered wafer can be processed using the teachings describedherein to introduce strain inducing regions for the express purpose oftuning the semiconductor properties into a desired state.

Furthermore, each individual device could be individually tuned via anon wafer trimming procedure, using for example ion implantation througha spatially selective mask or via focused ion-beams. Conversely, devicescan also be formed wherein the strain inducing region across a portionof an active semiconductor is dynamically alterable. For example,physical deflection or distortion of a semiconductor film in the form ofa two dimensional membrane can also modify the optical and electronicproperty of the active semiconductor. For example, a p-i-n homojunctionis formed into a device having a spatial portion that is a membrane. Themembrane can be distorted in a plane parallel to the semiconductor filmso as to laterally stretching the membrane or compressing the membranein one or two dimension in said plane and thereby introduce biaxial oruniaxial strain. Alternatively, the membrane can be deflected in adirection that is normal to the plane of the semiconductor filmresulting in a three dimensional strain inducing region. Furthermore,the membrane can be pre-stressed using a strain inducing region.

Portions of the following disclosure additionally describe a process fortuning the wavelength of light generated by a light emittingsemiconductor junction using a strain inducing procedure that isdisassociated from the selection of materials for the light emittingsemiconductor junction. This strain is induced after the material forthe junction has been selected, and the junction itself has been formed.In specific instances, the strain is induced through a global waferprocessing step. The following disclosure also describes procedures foriteratively selecting the material for the junction and the degree ofstrain induced in the junction. In specific instances, theaforementioned disassociation of the selection of material and theinduced strain facilitates these iterative procedures as they providetwo degrees of freedom for selecting the polarization and wavelength ofthe generated light. Furthermore, instances where the strain is inducedafter the junction is formed facilitate these iterative proceduresbecause the effect of the induced strain can be separately measured andapplied to the next iteration of the procedure.

As described with reference to FIG. 1a , the material selected to form alight emitting semiconductor junction has a direct impact on both thepolarization and wavelength of the generated light for a given devicestructure. Indeed, the band gap of the selected material is perhaps themost critical factor in determining the energy of the generated light.However, the introduction of strain to a light emitting semiconductorjunction can also alter the wavelength of light generated by thejunction. Introducing strain to a device will alter the crystalstructure of the material and will therefore alter the band gap. As aresult, related approaches have attempted to tune the light generated bya semiconductor junction by layering different composition semiconductormaterials with variant crystal structures in order to tune the opticalemission wavelength. The layered materials seek to minimize the inducedinternal strain as the dissimilar crystal structure films aresequentially deposited layer-by-layer on rigid substrate of fixedcrystal structure. The deposited films, if different in composition tothe substrate, can either stretch or compress in the plane of the layersin order to form atomic bonds and minimize crystalline defects such asmisfit dislocations.

The crystal structure mismatch must be carefully managed and limited inlayered dissimilar crystal structured stacks during formation; otherwisethe epitaxial manufacturing process will have much lower yields causedby dislocation defects generated by this internal crystal structuremismatch strain. It is also generally more challenging to arbitrarilycontrol the multiple compositions of heterogeneous junctions as comparedto epitaxial growth of a single composition junction. This is particularimportant for group-III Nitride semiconductors wherein the compositioncontrol and layer thickness is challenging for application to high Al %group-III Nitrides applied to deep ultraviolet operation.

Referring to FIG. 2, a light emitting device 150 operation is shown. Twoelectrical terminals 145 and 130 connect the electrical portion of thesemiconductor hetero-junction or homo-junction 140. A preferred device150 is a diode formed via p-n or p-i-n electrical conductivity regions.The external electrical terminals 145 and 130 typically form an internalresistance 135 due to non-ohmic behavior of the metal to semiconductorjunctions. Light that is internally generated by the diode 140 isoptically coupled through the semiconductor active region 155 andthrough an optical coupler 160 forming the out-coupled optical energy165. The optical coupler 160 can be optical matching layer or layersmade of transparent materials with a lower refractive index than thehost semiconductor active region 155. Optical coupler 160 can also be aphysically patterned structure such as a photonic band gap material,having the properties of minimizing the internal reflection losses ofphotons generated from within the region 155 so as to increase theout-coupled light power efficiency.

Fundamental to deep ultraviolet light emitting devices is the choice ofsemiconductor to achieve the desired wavelength of operation. Asemiconductor electronic bandstructure and electrical conductivity canbe intentional altered by three methods: (i) semiconductor composition;(ii) induced strain; and (iii) impurity type doping. The electronic bandstructure of a semiconductor can be altered by internal strain(typically due to hetero-epitaxy) and external strain (by theapplication of external stressor to the semiconductor). Wurtzitesemiconductors form the most technologically mature materials for theformation of deep ultraviolet light emitting diodes. Wurtzite widebandgap semiconductors such as the group-III Nitrides, have aparticularly important property. The wurtzite band structure for high Al% group-III Nitride compositions is particularly sensitive to internaland external strains imposed upon the crystal. Fundamentally, singlecrystal structured wurtzite can be manipulated to obtain unique opticalenergy and optical polarization properties by careful choice ofcomposition and strain. Unfortunately, the composition of a group-IIINitride semiconductor required for operation in the desirable opticalwavelength range of 200-280 nm is limited for vertical emissionstructures.

FIG. 3A shows the wurtzite crystal atomic arrangement of atomscomprising a two atom species composition of the form of wz-AB. The twoatomic species comprising the ionic crystal is formed for example, byanions of nitrogen and cations of a group-III metal. For example, themetal can be chosen from Al, Ga or In. Both the nitrogen 215 and metal220 atoms form polar bonds 260 as shown in FIG. 3A. The fundamentalrepeating unit of interest herein is the unit cell represented by theatomic structure of FIG. 3A. The wurtzite arrangement of group-IIINitride results in a hexagonal symmetry of the unit cell which can berepresented by the simpler geometry shown in FIG. 3B. That is, let thedetailed atomic arrangement of the unit cell in FIG. 3A be representedby the hexagonal column of FIG. 3B. It is found empirically that thewurtzite crystal structure can be epitaxially deposited layer-by-layerwith high crystalline perfection by growth along a direction 270. Thisenables the alternating stacking sequence of A-B-A-B- . . . , and so on,along the crystal growth direction 270 for the formation of the wurtzitecrystal. The unit cell represented by FIGS. 3A and 3B shows the socalled c-plane face hexagon 210 having equal side of length 205separating the cations 220 (for example Al atoms on the c-plane). Thevertical height along the growth direction 270 for the fundamental unitcell is given by the length 250. For example, the bulk wurtzite AlNcrystal can now be fully specified by a crystal structure havingdimension 205 given by 3.112 angstroms and dimension 250 given by 4.982angstroms (at room temperature and no external or internal strain).

An idealized epitaxial layer of single crystal structured wurtzitesemiconductor, such as AlGaN, is formed simultaneously as a sheet ofinterconnected and adjacent unit cells 500 in FIG. 5. As shown in FIG.4, top view c-plane surface 230 comprising close packed wurtzite unitcells 210. The metal atoms 420 are shown on the topmost plane and thenext layer below comprises nitrogen atoms 425 (small black spheres). Inthe plane of the layer there exist three high symmetry crystaldirections, shown as 405, 410 and 415. There also exist higher ordersymmetry directions within the wurtzite crystal. The high 3-foldrotation symmetry of atoms within the c-plane directly determines theelectronic band structure of wurtzite crystal.

FIG. 5 shows a perspective drawing of calculated atomic positions ofAlN. The interconnected atoms for Al and N are illustrated to show thenitrogen polar bond formed along the c-axis 305 by the N 215 and Al 220atoms. The c-plane surface 230 comprising a plurality of hexagonal unitcell faces 210 that can be defined by the crystal axes 315 and 310. FIG.5 represents a three dimensional slab of wurtzite material orientedalong a growth direction 305. The lateral extent of the crystal alongdirections 315 and 310 is substantially determined by the availablesubstrate surface area. The vertical extent of the crystal alongdirection 305 is determined by the growth rate and time of depositionfor a particular deposition process. Using optimized growth conditionsfor the wurtzite crystal, the anions and cations comprising the wurtzitesemiconductor are self-assembled layer-by-layer into the idealizedcrystal shown in FIGS. 2, 3, 4 and 5. Non-ideal crystal formations occurdue to atomic bond misfit dislocations, layer stacking faults and thelike. Such non-ideal occurrences can be tolerated within the crystalstructure without detracting from the fundamental electronic bandstructure property sought by the idealized structure. However, if thecrystal imperfections begin to dominate the idealized picture, then theelectronic band structure will deleteriously suffer and the utility ofsemiconductor material will diminish. Many of the approaches describedherein alleviate this problem because misfit dislocations are mitigatedand electronic performance is modified by a post growth stressor method.

Understanding the unique properties of single crystal wurtzitesemiconductor structure enables description of another process that canapplied in combination with approaches described herein. The formationof specific conductivity type regions occurs by substitutional doping ofmetal sites within the wurtzite crystal and is achieved in simplest formby co-deposition of impurity atoms simultaneously during the filmformation process. However, other methods are possible, such as impuritysheet doping and ion implantation technique.

FIG. 6 shows a 3D wurtzite slab grown along a direction 305 formingso-called c-plane 230 oriented films. The structure of FIG. 6 comprisesthree distinct layered regions labelled as layers 605, 610 and 615. Ann-type layer 605, a not intentionally doped (NID) or intrinsic layer 610and a p-type 615 layer are shown forming the layered p-i-n device formedusing a single wurtzite semiconductor host composition. It is understoodthat impurity atoms randomly populate a portion of the respective dopedlayer metal sites forming an activated doping concentration. The device600 forms a vertical p-i-n homojunction.

Alternately, a slab of wurtzitic material can be formed into a lateralp-i-n homojunction by diffusion, impurity ion implantation or waferbonding to form the device 700 shown in FIG. 7. An n-type region 515laterally connects an intrinsic or NID region 520 to a p-type region525. The epitaxial thickness 510 and device width 505 are now definedpost epitaxial growth of the wurtzite semiconductor. FIGS. 8 and 9 showthe vertical p-i-n homojunction 800 and lateral p-i-n homojunction 900formed along a crystal growth direction 305 and on a substrate 810.

Electrical device structures implemented in a wurtzite semiconductor arenow discussed. FIGS. 10 and 11 disclose the p-n and p-i-n homojunctions,respectively formed using group-III Nitride material. Specifically, agroup-III Nitride semiconductor composition of x=0.7 (i.e., 70% Al)Al_(x)Ga_(1-x)N ternary alloy is selected to highlight the uniqueelectronic band structure issues facing light emitting diode operationin the deep ultraviolet wavelength regime. FIGS. 10 and 11 disclose thecalculated spatial band structures of the diodes showing the variationin conduction band edge energy E_(C)(k=0,z) and the three zone centervalence bands labeled as the crystal-field split (CH) bandE_(CH)(k=0,z), the heavy-hole band E_(HH)(k=0,z) and the light-hole bandE_(LH)(k=0,z).

Referring to FIG. 8, the layered p-i-n diode that is formed along thec-axis 305 is parallel to the growth direction, z labelled in FIGS. 10and 11. The valence band labels reflect the energy-momentum dispersionproperties and have a distinct symmetry type that is directly tied tothe optical polarization processes that occur. Electrons 1005 areinjected from the n-type region, and holes 1010 are injected from thep-type region. Recombination of an electron and hole within a directband gap semiconductor is represented as pure vertical transitions withrespect to the spatial band structures shown. A photon 1015 and 1115 aregenerated by electronic recombination of an electron and a holeannihilating to form a new particle being the photon. As the growthdirection is along the c-axis of the wurtzite crystal 305, the layersare formed upon the c-plane 230 of crystal and the energy ordering ofthe valence bands E_(CH), E_(HH) and E_(LH) are as shown in the banddiagrams of FIGS. 10 and 11. The k=0 representation of the bandstructure energy-momentum dispersion is sufficient to describe theoptical processes with the devices of FIGS. 10 and 11. The highest lyingvalence band in x=0.7 (i.e., 70% Al) Al_(x)Ga_(1-x)N ternary alloywithout any strain is the CH band. As such, photons can couple to theelectronic transition between the conduction band and the CH bandefficiently with optical polarization substantially perpendicular to thegrowth direction, z. This mode of operation is ideal for waveguide typestructure wherein the light propagates substantially parallel to theplane of the layer (labelled as transverse magnetic polarization TM),however, it is disadvantageous for vertical type emitters (i.e., lightpropagating parallel to growth direction or substantially perpendicularto the plane of the layers).

Strain can be used to engineer the optical properties of wurtzitesemiconductors for utility as light emitting devices. A method tointroduce strain in wurtzite semiconductors is via the method depositinglattice mismatched semiconductors to form heterojunctions. By changingthe semiconductor composition of group-III Nitride alloy, the bandstructure can be selected. A fundamental limitation, discussedpreviously, for the heteroepitaxial growth of layered semiconductors isthat of managing the misfit strain energy and avoiding deleteriouscrystalline dislocations. The critical layer thickness for group-IIINitride semiconductors places a limit of 1-100 monolayers of materialwhich can be epitaxially deposited under elastic deformation (i.e.,achieving dislocation free interfaces) for in-plane strains of the orderof 0.1 to 1%. This limitation in thickness of material renders opticalstructures impractical or deficient in optical thickness and thuslimited devices performance and efficiency.

The epitaxial growth of different semiconductor compositions to form aheterojunction can be explained with reference to FIGS. 12A and 12B.Consider a top view of the c-plane oriented film comprising a ternarysemiconductor of AlGaN as shown in FIG. 12A. The c-plane 230 shows ametal terminated surface comprising Al and Ga atoms forming the ternaryAlGaN wurtzite crystal structure. The crystal direction 1210 is similarto the direction 415 of FIG. 4. FIGS. 12A and 12B further show labelledAl metal atoms 1220 and 1230 and Ga metal atoms 1225 and 1235. FIG. 12Bshows the vertical crystal structure along the growth direction 305 withthe uppermost c-plane 230. The upper c-plane metal atoms are labelledshowing the orientation of direction 1210 in FIG. 12A.

Referring to FIG. 12B again, a representation of the heterostructureformed by an Al_(0.5)Ga_(0.5)N layer 1250 deposited elastically on anAlN layer 1270 is shown. It is understood that many more monolayersalong the growth direction 305 can occur, but only one monolayer of eachis shown for clarity.

The principle of elastic deformation of lattice mismatched wurtzitematerials is disclosed in FIGS. 12A and 12B by understanding that theAl_(0.5)Ga_(0.5)N alloy has unstrained unit cell crystal dimensionswhich are larger than the underlying AlN layer. That is, the dimension205 of FIG. 3A is larger for Al_(0.5)Ga_(0.5)N compared to AlN.Therefore, in order for the Al_(0.5)Ga_(0.5)N epilayer to form adislocation free interface 1260 on the AlN surface, there needs to be acontraction of the Al_(0.5)Ga_(0.5)N unit cell parameter 205. Within theelastic deformation regime, the unit cell volume is conserved andtherefore the Al_(0.5)Ga_(0.5)N unit cell deformed laterally toaccommodate the in-plane lattice constant mismatch at 1260 andcompensates by elongating the unit cell parameter 250. The ability ofthe Al_(0.5)Ga_(0.5)N layer to elastically deform in this manner islimited by the thickness 1250 of alloy deposited. Beyond the CLT ofAl_(0.5)Ga_(0.5)N deposited on AlN, the strain energy at the interface1260 exceeds the competing energy to form a dislocation (missing bondsat the interface 1260). Looking again at FIG. 12A at the top view of theAl_(0.5)Ga_(0.5)N c-plane 230, it is evident that hexagonal unit cellsparameters 205 are matched for both the Al_(0.5)Ga_(0.5)N and AlNmaterial. This uniform lateral contraction of the unit cell parameter205 occurs in all directions within the plane of the layer and is calledtri-axial strain (refer to the in-plane high symmetry directions 405,410 and 415 of FIG. 4). That is, a preferred direction within the planeof the layers can not be manipulated in preference to the other in-planedirections for the explicit purpose of introducing a preferred directionof strain.

Conversely, tensile tri-axial in-plane strain can be introduced in alattice mismatched wurtzite film as shown in FIGS. 13A and 13B. FIG. 13Ashows the c-plane 230 of a wurtzite semiconductor, for example AlN, withthe unit cell 210 identified. If the wurtzite film is then stretchedequally by tensile in-plane forces 1305, 1310, 1315 and 1320 within theplane, as shown by FIG. 13B and as would occur by epitaxial depositiononto a larger underlying lattice constant material, then the unit cellwould symmetrically increase in size to 1330. This is the only elasticdeformation process that can occur with high crystal quality growth forthe epitaxial deposition of mismatched lattice constant wurtzite alloys.

There are suitable methods for introducing so called uniaxial strain tothe wurtzite crystal as shown in FIGS. 14A and 14B. Again, a top view ofa c-plane oriented film 230 is shown with topmost metal terminations 220in FIG. 14A. FIG. 14B shows a uniaxial compressive stress applied in theplane of the layer along directions 1420 and 1425. The wurtzite crystalresponds within the elastic deformation regime by elongating along thein-plane crystal directions 1405 and 1410 such that it encompassesregion 1430 of c-plane 230.

FIG. 15 shows the 3D representation of a portion of the c-plane oriented230 wurtzite crystal structure having uniaxial stress applied along thepreferred crystal axis 315 to achieve a compressive in-plane strain. Theuniaxial stressors 605 and 610 provide equal and opposite directionalstrain to the wurtzite crystal slab. One or more uniaxial stressors canbe supplied to the wurtzite crystal and can be along arbitrarydirections. In preference, uniaxial, bi-axial and tri-axial stressorsare claimed herein which can be directed across an active regioncomprising a wurtzite semiconductor. Another approach involves at leastone in-plane stressor disposed across a portion of a wurtzitesemiconductor and substantially perpendicular to the c-axis of thewurtzite crystal. Another approach involves at least one in-planestressor disposed across a portion of a wurtzite semiconductor andsubstantially parallel to the c-axis of the wurtzite crystal. Anotherapproach involves at least one in-plane stressor disposed across aportion of a wurtzite semiconductor directing a strain field that hascomponents parallel and perpendicular to the c-axis of the wurtzitecrystal.

FIGS. 16A and 16B show the effect of strain on the wurtzite bandstructure for the case of compressive uniaxial, bi-axial or tri-axialstrain. FIG. 16A shows the energy-momentum band structure of a directbandgap and high Al % group-III Nitride semiconductor having the valenceband ordering as shown. The CH band 1640 is the highest lying valenceband and thus optical processes are dominated by electron-holetransitions between the conduction band 1635 minimum and the CH band1640 maximum, which are coincident at k=0. Necessarily, symmetryarguments dictate that the dominant optical polarization of theEγ^(C-CH) transition will be of TM type relative to the c-axis of thewurtzite crystal.

The energy of the photon Eγ^(C-CH) that couples to the material 1605will be further reduced from the energy gap at k=0 by the exciton energyEX_(BE). For example, consider the lower portion 1605 of the devicehaving no externally applied stressor (ε=0) and composed ofAl_(0.7)Ga_(0.3)N. If the same alloy is then spatially strained suchthat an upper portion 1610 is subjected to uniaxial compressive stressas shown, then the energy-momentum band structure is modified as in FIG.16B. If sufficient in-plane strain is applied across material 1610 thenthe band structure can be transformed so as to reverse the energy orderof the valence band and result in the heavy-hole and light-hole beingabove that of the CH band. Symmetry again dictates that the material1610 becomes optically active to TE polarization coupling between theconduction band 1645 and the highest lying HH band 1650 at k=0.Therefore, the simultaneous effect of spatially modifying thebandstructure of material 1610 via compressive in-plane stressor ε<0produces an increase in the fundamental optical transition energyEγ′^(C-HH)(ε<0) relative to the strain free material below, and a changein the coupling of optical polarization to the energy bands. If region1610 is generating photons 1630 with TE mode Eγ′^(C-HH)(ε<0), forexample using a p-i-n or p-n diode configuration, then they can notefficiently couple to TM absorption within the lower portion 1605semiconductor. This is particularly advantageous for optical extractionefficiency of light from a device and is claimed herein.

Yet another example of how the band structure can be transformed isdisclosed in FIG. 17. Consider again a wurtzite material 1705 that isconfigured with strain-free energy-momentum dispersion bandstructure asshown in FIG. 17A. The optical activity is dominated by the k=0 energytransition between the conduction band 1735 and the highest lying HHband 1740 having TE-like energy Eγ^(C-HH)(ε=0). Application of, forexample, uniaxial tensile strain via stressors 1720 and 1725 to a region1710 will reduce the effective bandgap to Eγ′^(C-HH)(ε>0). Which willalter the bands to 1745 and 1750 respectively. Therefore, a photon 1730generated within active region 1710 can escape through the lower region1705 by virtue of having a smaller photon energy that is not absorbed inmaterial 1705.

Group-III Nitride alloys are of particular interest. FIG. 18 shows yetone more example of how the unique band structure of the wurtzitecrystal symmetry semiconductors are controlled by the approachesdescribed herein. FIG. 18 shows the valence band energy at the center ofthe Brillioun zone (k=0) of the CH, HH and LH bands as a function ofuniaxial, biaxial or tri-axial strain applied perpendicular to thec-axis (305). For the AlGaN alloy composition having x=70%Al_(0.7)Ga_(0.3)N, which is of high utility for deep UV light emittingdiodes, the bandstructure is dominated by the valence band energyordering. Clearly, the application of in-plane stress to the AlGaNmaterial results is a dramatic transformation in both fundamental bandgap and the optical polarization properties. For in-plane compressivestrain exceeding ε≦−1.25% the TM-like material transforms into TE-likebehavior. The fundamental bandgap (i.e., the lowest energy photon thatcan be emitted or absorbed) is then determined by the energy transitionfrom the conduction band minimum and the valence band maximum. As boththe valence band maximum and conduction band minimum are directlyaffected by the in-plane strain, the optical gap varies as shown in FIG.19. Clearly, compressive in-plane strain results in an increase in thefundamental optical transition 1905, whereas in-plane tensile strainresults in a reduction of the fundamental gap 1910 relative to the zerostrain bulk case (ε=0). The implications for these trends are summarizedby the selected alloys disclosed in FIG. 20.

FIG. 20 discloses the optical properties of group-III Nitride materialsas a function composition and applied external in-plane stressors. Threerepresentative Al_(x)Ga_(1-x)N materials subjected to in-plane stressfor cases of x=1.0, 0.7 and 0.5 are calculated using a full 8×8 k.pHamiltonian assuming symmetric in-plane stressors. The vertical axis2010 of FIG. 20 represents the fundamental band gap in units ofwavelength (nm) as a function of the in-plane strain %. The in-planestrain 2030 is defined as the ratio of the in-plane lattice constanta_(e) dilation or expansion relative to the strain-free in-plane latticeconstant a₀ case, strain %=100×(a_(e)−a₀)/a₀, whereas ε=(a_(e)−a₀)/a₀.The dotted line 2015 shows the crossover from TE 2020 into TM 2025 andvice versa as a function of in-plane strain for each AlGaN composition.The higher Al % compounds requires a larger in-plane compressive stress(ε<0) to achieve TE optical polarization behavior. Using the trends ofFIG. 20, a device can be engineered for a desired optical specificationof emission wavelength and optical polarization performance by selectingthe material composition and the stress state.

Implementing a predetermined stress state enables a homojunction andheterojunction device to operate optimally for a given deviceconfiguration. For example, consider the layered p-i-n homojunctiondevices disclosed in FIGS. 21A and 21B for the case of compressivein-plane uniaxial stress and FIGS. 22A and 22B for the case of tensilein-plane uniaxial stressor. The spatial k=0 band structures for thep-i-n homojunction diode composed of Al_(0.7)Ga_(0.3)N are shown in FIG.21A for compressive ε=−2.5% and FIG. 22A for tensile ε=+2.5% uniaxialstress applied perpendicular to the c-axis 305 and along the <10-1-1>wurtzite axis. Referring to the strain free p-i-n of FIG. 10, it isapparent by the teaching herein that the valence band order is reversedto provide TE-mode optical coupling as the highest lying valence band istransformed from the CH band to the HH band by the application ofuniaxial compressive stress. FIG. 21B depicts the uniaxial compressivestressors 2130, 2135, 2140 and 2145 applied perpendicular to the growthdirection 2125. The compressive stress is uniformly applied to thecontact planes defining the device formed by epitaxial layers of p-type2150, i-type 2155 and n-type 2160 material. The layers are assumed to beformed upon a c-plane according to the approaches described herein. Thefundamental or lowest energy optical emission 2170 is then set to occurpreferentially along the growth direction 2125.

Conversely, FIG. 22A shows the band structure for tensile uniaxialstress across the p-i-n Al_(0.7)Ga_(0.3)N homojunction diode. The stressis again assumed to be applied along <10-1-1> wurtzite axis. Tensilestress exacerbates the energy ordering above the strain free case andmoves the CH band to a lower energy position through the structurecausing the fundamental transition to remain TM-like. FIG. 22B shows thein-plane uniaxial stressors 2230, 2235, 2240, and 2245 appliedperpendicular to the c-axis growth direction 2125 pulling uniaxially thep-i-n Al_(0.7)Ga_(0.3)N homojunction diode along the <10-1-1> wurtziteaxis. The optical emission of the lowest energy photons 2220 will beTM-like and tend to be emitted in the direction described by vector2270.

Having described fully the methods for introducing modification ofwurtzite optical and electronic properties through the action of atleast one stressor, certain preferred devices will now be described, butthe approaches disclosed herein are not limited to these specificexamples disclosed. There are distinct types of external stressors whichcan be implemented using the conventional and new fabrication techniquesdisclosed herein. Certain approaches described herein utilize in-planestressors applied to wurtzite materials that are directed preferentiallyalong at least one in-plane crystal axis.

A symmetrical in-plane stressor is disclosed in FIGS. 23A, 23B, 23C, 24and 25 showing cylindrical symmetry. FIGS. 23A-C show a cross-sectional3D view of p-i-n homojunction diode deposited or formed on a substrate2305. The p-i-n diode comprises n-type layer 2310 that has a portionformed 2315 and 2320 into the active region of the diode. An optionalNID region 2325 separates a p-type layer 2330. The p-i-n layeredstructure is preferentially formed first followed by a patterning stepto form the cylindrical diode structure. Metallic and or ohmic contact2340 is formed on n-type region 2310 and upper metallic and ohmiccontact 2360 is formed on a portion of the p-type material that isetched 2330. A cylindrical stressor 2350 is spatially formed bydeposition or attachment onto the sidewalls of the p-i-n diode. Thestressor can be deposited conformably or via a process which results insubstantially uniform material that has a large intrinsic stress. Thestressor can also be composed of a material dissimilar to the diodehomojunction material and have a large dissimilarity in thermalexpansion coefficient. For example, sapphire, aluminum oxide, siliconnitride, silicon oxide, diamond, diamond-like carbon and others arepossible. The stressor may also be composed of a multilayered sequencecomprising an insulating material separating another highly stressedmaterial from the p-i-n sidewall. For example Al₂O₃ can be used as aninsulative material separating a highly stressed conductive or metallicmaterial, such as tungsten.

FIG. 24 illustrates a cylindrical and radially symmetric 2510compressive in-plane stressor 2505 applied to the p-i-n diode 2415. Thestressor can contact at least a portion of the p-i-n diode sidewall.FIG. 25 illustrates a cylindrical and radially symmetric 2510compressive in-plane stressor 2505 applied to the p-i-n diode 2415. Thestressor can contact at least a portion of the p-i-n diode sidewall.Therefore, FIGS. 21 and 22 represent uniaxial in-plane stressors appliedacross a p-i-n diode whereas FIGS. 24 and 25 describe bi-axial andtri-axial in-plane stressors applied across the p-i-n diode.

Other multi-axis in-plane stressors are possible as shown in FIGS. 26Aand 26B. FIG. 26A shows a p-i-n homojunction diode fabricated into ahexagonal mesa structure. The n-type region 2605, i-type region 2610 andp-type region 2615 and the contacts 2620 and 2630 complete the twoelectrical terminal diode. The in-plane stressor 2360 is shown in FIG.26B as a conformal coating but can also extend laterally beyond theconfines of the lower contact 2620.

FIGS. 27 and 28 show the device of FIG. 26B interconnected into aplurality of devices. FIG. 27 show the plan view of 7 devicesinterconnected in parallel by bus contacts 2745 and 2740. The sevenp-i-n devices 2750, 2705, 2710, 2715, 2720, 2730 and 2735 have in-planestressors formed. FIG. 28 shows the multi-diode device in detail withwafer scale fabricated stressors.

Uniaxial in-plane stressors can also be applied to multi-diodestructures as shown in FIG. 29. Substrate 2305 includes buffer or n-typematerial 2315. A plurality of rectangular p-i-n diodes are fabricatedfrom the p-i-n epilayer materials. The n-type regions 2905, i-typeregion 2910 and p-type region 2915 after being etched are formed intorectangular fingers of lateral width 2955 and length 2960. The lateralp-i-n fingers have the intervening space separating adjacent mesasfilled or formed with lateral stressor regions comprising stressormaterials 2920, 2925, 2930, 2935, 2940 and 2945. Each stressor regioncomprises three materials, for example regions 2935, 2940 and 2950. Ifthe two stressor regions sandwiching a p-i-n finger induces acompressive stress then lateral uniaxial stress is applied to the saidfinger. The stressor region can for example, comprise stressor materials2935 and 2945, and may include a contact material capable ofelectrically connecting the lower conductivity type region of thediodes. For example, each stressor material may comprise insulatingmaterials and a metallic plug that can simultaneously act as a stressorand n-type contact bridge.

FIG. 30 shows the action of the stressor regions providing uniaxialstrain across a plurality of rectangular p-i-n fingers. The expansion ofeach stressor region exerts compressive stress against the respectivesidewalls of the p-i-n mesa. A further optional feature of the stressorregions incorporating a conductive bridge from the surface to a lowern-type region is shown in FIG. 31. Interdigitated contacts 3115 contactthe conductive bridge down to the lower n-type region. Interdigitatedcontacts 3115 connect in parallel the p-type regions of all the p-i-nfinger mesas. The interdigitated electrical structure interconnects aplurality of lateral p-i-n mesa diodes in parallel.

A plurality of stressor regions can also be enabled to exhibit multistressor components along a vertical axis, for example perpendicular tothe plane of the layers. FIG. 32 shows schematically a verticalstructure comprising a homogeneous material composition, for exampleAlGaN. The device is physically etched and or patterned so as to formthree distinct regions 3205, 3210 and 3215 with dissimilar strainstates. The lower portion 3205 can be a substrate and the region 3210enabled with tensile in-plane stressors 3220 and 3225. This is usefulfor reducing the energy of the optical emission region. The upper region3215 can be enabled with a different strain field, such as, compressivein-plane stressors 3230 and 3235. Region 3215 could be a p-type AlGaNlayer that is desired to have compressive stress to improve the p-typeactivation by virtue of the strain induced valence band energy shiftrelative to the ionized impurity.

Simple structures using uniform strain fields have been described.Sub-micron scaled devices comprising wurtzite materials offer furtheropportunities for advantageous utilization of strain. It is anticipatedthat sub-micron scaling using silicon CMOS style fabrication techniqueswill enable greater control and opportunity for bandgap engineering ofhomojunction devices. It is further anticipated that strain engineeredhomojunction wurtzite devices fabricated using nanometer scaledimensions will be competitive and potentially outperform heterojunctiontype compound semiconductors. That is, complex heterogeneousmultilayered wurtzite epilayer structures may eventually be replaced byextremely high quality and low defect density homojunction epilayerswhich are then formed into nanoscale devices. Anticipating thesetechnology advancements, the following disclosure describes how thewurtzite bandstructure can be engineered using uniaxial stressors andsize effects.

FIG. 33 shows an ideal bulk-like AlGaN mesa structure sandwiched by twocompressive uniaxial stressors. The wurtzite AlGaN material is formedepitaxially on an AlN substrate or buffer. The lateral stressors aredirect along the <10-1-1> in equal and opposite amount perpendicularlyagainst the sidewalls of the AlGaN mesa. Straight and ideal sidewalls(0° slope) are shown, with the lateral stressors introduced by selectivearea regrowth of material. A topmost passivation layer of Al₂O₃ servesto conserve shear flow and protect the device from the externalenvironment. The mesa dimensions in the x and z directions are 250×250nm and chosen to be compatible with conventional Si CMOS 0.25 umlithography. The z-direction is selected along the c-axis and thus theAlGaN epilayer is grown with c-plane oriented growth. The unique elastictensor of the wurtzite crystal provides advantageous properties for sucha uniaxially stressed AlGaN mesa. FIGS. 34, 35 and 36 show thecalculated 2D stressor elements e_(xx)(x,z), e_(xz)(x,z) and e_(zz)(x,z)due to the uniaxial compressive stressors, respectively. The electronicband structure is a sensitive function of the strain tensor and thus theconduction and valence bands will be spatially modified in a non-trivialmanner.

FIGS. 34, 35 and 36 disclose the strain tensor map within the AlGaN mesaand clearly demonstrate the non-uniform effect of the uniaxial stressor.Vertical through the mid-section of the mesa along the z-direction it isevident that large changes in elastic strain occur. The calculatedeffect on the spatial distribution of the conduction and valence bandswithin the AlGaN mesa region is disclosed in FIGS. 37, 38, 39 and 40 forthe conduction band minimum, CH, HH, and LH bands, respectively. FIG. 41shows the actual 2D spatial variation in the conduction to LH transitionenergy within the mesa structure. The effective bandgap of the mesaalong the mid-section along the z-direction is shown to be larger at thetop compared to the bottom. This effect is particularly useful for ap-in homojunction diode wherein the optical generation region occurs inthe lower portion of the mesa and the p-type region is activated by theincrease in effective bandgap. Therefore, it is shown that ahomojunction p-i-n device using wurtzite materials can be bandgapengineered using in-plane lateral stressors.

The ideal straight sidewall mesa of FIG. 33 can also be modified toprovide an angled sidewall as shown in FIG. 42. A sidewall of 20° fromthe vertical is modelled forming the trapezoidal AlGaN mesa. Againcompressive uniaxial stressors are applied. Periodic boundary conditionsare modelled in the x-direction to simulate a plurality of such devicesas shown for example in FIG. 30.

The calculated 2D spatial stress tensor components for the angledsidewall mesa are shown in FIGS. 43, 44, and 45 for e_(xx)(x,z),e_(xz)(x,z) and e_(zz)(x,z), respectively. Compared to the straightsidewall case, the angled sidewall shows larger effect for bandgapmodification near the top of the mesa. For the case of p-i-n diode withp-type as the topmost region, the angled sidewall device would providean improved current aperture effect further improving hole injectionefficiency into the center portion of the mesa.

Therefore, the geometry of the mesa and the application of the in-planestressors can improve the p-i-n device performance for application toDUV light emitters. FIGS. 46 and 47 further show the calculated 2Dspatial variation of the conduction band and HH band within the taperedsidewall mesa under the influence of externally applied uniaxialcompressive stressors.

Devices formed using some of the approaches described herein can provideadvantageous device performance above those achieved in the prior artusing wurtzite materials. As the methods for design of external appliedstressors for application to wurtzite crystal structured materials havebeen fully enabled and described in detail, the manufacture ofexternally applied stressors to wurtzite semiconductors is nowdisclosed.

Referring again to FIG. 1B, examples of single crystal and transparentsubstrate 112 are crystalline sapphire (i.e., crystalline aluminiumoxide Al₂O₃), wide bandgap metal-oxides, crystalline native group-IllNitride substrates such as AlN and GaN. Ternary substrates ofaluminium-gallium-nitride (Al_(x)Ga_(1-x)N where 0<x<1) are alsopossible.

Light generated at point 113 in active layer 111 will disperse in alldirections, but the crystal structure and chemistry of active layer 111will produce an emission pattern that favors certain directions.Emission patterns favoring direction 114 are said to favor thetransverse electric or s-propagation direction. Emission patternsfavoring direction 115 are said to favor the transverse magnetic orp-propagation direction. For various applications of semiconductor lightgenerators, both the polarization and wavelength of light needs to betuned to a desired value, and that value needs to be maintained within apredetermined acceptance window across the expected operating conditionsof the device. Techniques for adjusting the polarization and wavelengthof light generated by semiconductor light generators are thereforeimportant for both the design and operation of those devices.

Once the wafer scale epitaxial material formation process is completed apost epitaxy selective area strain inducing process can be applied tothe wafer globally to further modify the band structure of thesemiconductor. Significant cost savings can be realized as every deviceon the wafer can be tuned in a single processing step by usingsemiconductor material of lower material variation and high crystallinequality. In this regard, processes that are conducted prior tosingulation of the individual die from a wafer provide significant costbenefits. As the cost of semiconductor devices is intimately tied to theamount of processing time required to fabricate each die, approachesthat process and tune wafers in batched format are less costly to thosethat apply to individual dice.

Finally, the usage of processing methods to induce selective area andstrain provides additional benefits over methods that induce straininternally within the epilayer formation process. This epilayer materialrequires less growth complexity and can improve at least one of the mostimportant crystal structure properties, defect density. In fact, this iswhat historically occurred to the silicon microelectronics industry.During early Si microelectronics development both device fabrication andadvanced material formation methods were investigated. Eventually, aconsensus for improving Si device performance was found to require animprovement in bulk Si crystal quality. It was the single most importantattribute for large volume yield improvement. Post epitaxial crystaldevice formation leveraged advanced lithography and device formationtechniques, such as oxidation and ion implantation. Approaches disclosedherein anticipate a similar trend in compound semiconductors and morespecifically wide band gap semiconductor applicable for optical emissivedevices. That is, simplifying the epitaxial structure and improving thestructural quality is a key aspect for increasing the group-III Nitrideyield and device performance. Approaches that induce strain as a bandstructure tuning methodology independent to and after the activesemiconductor deposition process enables access to technologicallymature Si CMOS compatible fabrication processes.

Numerous types of strain tuned light emitting semiconductor junctiondevices can exhibit some or all of the benefits described above. Thedevices can be formed using a bulk-like epilayer or substrate thatcomprises the material used for the junction. The function of bulk-likesemiconductor could then be fabricated into a wider array of devicesusing standard fabrication procedures. That is epilayer can be tailoredin part using device fabrication methods rather than relying solely onthe epitaxial structure itself. This implementation highlights anotheradvantage to partly disassociating the material selection for thejunction and the light generated by the junction. Providing greaterflexibility for selection of the material used to build thesemiconductor junction can lead to significant cost savings asfabrication procedures that are unavailable for devices with exoticsubstrates are generally less expensive due to economies of scale and agreater degree of industry experience.

Referring again to FIG. 23A and FIG. 23B, strain is induced in device2300 through a region of set straining material 2350. In this situation,the region of set straining material is not a contiguous region. Asshown, breaks in the region of straining material are made to provideelectrical contacts 2350 and 2360 to the device. Additional breaks inthe region may be formed elsewhere to accommodate other features of thedevice. The region of set straining material 2350 lays outside thecurrent paths that includes the first electrical contact formed by theelectrode 2360 on the first side of the junction above p-type layer2330, and the second electrical contact formed by either of theelectrodes 2340 that are coupled to n-type layer 2320. Furthermore, theelectrodes 2340 and 2360 provide current paths for the light emittingjunction that are independent of the region of set strain material, andthe region of set strain material is not on the current path between anyof the electrodes. As a result, current can be provided to the junctionand the light emitted by the junction can be measured prior to theintroduction of the set strain region. This measurement could beconducted as a wafer level process using test equipment that applies acurrent to each of the many junctions that may exist on a single die. Asa result, the recipe and final characteristics of the region of setstraining material can be adjusted and calibrated to affect the opticalpolarization and emission wavelength of the particular junctions on thedie. In specific instances, the strain inducing region can alter theoptical polarization of the devices so that their polarization isopposite before and after the introduction of the set strainingmaterial. In the particular situation of a DUV LED, the ability of theset strain inducing region to alter the optical polarization fromtransverse electric to transverse magnetic provides a particularlyappealing benefit in that higher wavelength light can be created withless strain in the device.

FIG. 23C shows yet another example of fabricating the in-plane stressorof device shown in FIG. 23A. The stressor material 2350 can be formed asa blanket and or conformal layer after the p-i-n mesa is patterned. Thestressor material can also function as a passivation material to theactive semiconductor material. Alternately, the stressor layer 2350 canbe formed by consuming a portion of the immediate surface of the activesemiconductor after the p-i-n mesa is formed. For example, a group-IIINitride can be converted via selective oxidation into a group-III oxideor oxynitride.

The region of set strain material in device 2300 can also be apassivation layer 2350. Passivation layer 2350 is deposited after thejunction has been created. The passivation layer can be deposited insuch a way that it creates a tensile or compressive force across thesurface of the entire wafer and thereby induces strain in thesemiconductor junction. The strain induced in the junction can beuniaxial, biaxial or tri-axial, as described herein. The depositionconditions and the material deposited can affect whether the passivationlayer creates a tensile or compressive force. In fact, the same materialcan produce either kind of force depending upon the conditions by whichit is deposited (i.e., if it is deposited at a temperature below regularoperating temperature or above regular operating temperature). Likewise,the thickness of an inherently strained film deposited or formed uponthe junction can also be used as a further mechanism for strain tuningin that a similar layer will exert a different degree of force based onits thickness.

Numerous benefits are provided by introducing strain through passivationlayer 2350. In contrast to methods that involve heterogeneous materialselection with variant crystal structures, passivation layer 2350 doesnot induce strain through epitaxial crystal lattice mismatch and doesnot ideally affect the light generated by the device except through theinduction of strain.

Furthermore, the light generated by the semiconductor junction can bemeasured before and after the deposition of the strain inducing stressoror passivation layer because the device is substantially complete by thetime passivation layer 2350 is put into place. Also, after thepassivation layer is deposited, openings can be made in the passivationto reconnect to the control electrodes to measure the performance of thedevice again with the strain layer in place. As described below, thisfacilitates the iterative design of a desired junction and also improvesthe accuracy of manufacturing processes meant to minimize wafer to wafervariation. Finally, stressor layer 2350 is deposited globally across thesurface of a wafer such that the strain induced by the passivation layeraffects all devices on the wafer at the same time which provides costbenefits and simplifies the tuning procedure.

Strain can be introduced at multiple stages during fabrication; however,it is beneficial to introduce the stain towards the end of thefabrication cycle.

For example, the deposition of technologically mature amorphous siliconoxide and silicon nitride stressor films using plasma enhanced chemicalvapor deposition (PECVD), atomic layer deposition (ALD) and sputterdeposition processes can be used to form layer 2350 as shown in FIG.23C. Windows can then be opened in stressor layer 2350 to formelectrodes 2340 and 2360. Additional stressor layers can then bedeposited or formed to increase the desired stressor strength and type(namely, compressive or tensile). As a result, externally induced strainis achieved with minimal additional cost. Also, PECVD generally allowsfor tight global control of the deposited material and thereforeminimizes within-wafer non-uniformity and improves the ability of adesigner to select the right material. In general, processes that can beapplied globally on a wafer, but still allow control of the degree ofstrain at the device level, should be favored above those that do not.The aforementioned characteristics make PECVD stressor films well suitedto provide strain in a manner that is disassociation from the selectionof material for the active layer.

Another strain tuned light emitting semiconductor device can bedescribed with reference to wafer section 4800 in FIG. 48. An activediode 4825 composed of wurtzite semiconductor material is patterned intorectangular mesa as shown. The device is formed in semiconductormaterial similar to the epitaxial layer 4820, which is formed on asubstrate 4805. An optional surface stressor 4830 is shown on thetopmost portion of the device 4825. Furthermore, asymmetric and uniaxialstrain is induced into at least a portion of device 4800 through theformation of excavated trench regions 4850 in layer 4820. Furthermore,an undercut region 4840 is etched beneath at least a portion of theactive device 4825. These trenches 4840 can be etched deep into layer4820, and can further be backfilled with a contact material or stressormaterial.

Once the trenches are formed and optionally filled, a strain field isinduced in at least a portion of the junction structure 4825 by virtueof device having intimate epitaxial contact at regions 4815 while beingsuspended along the side 4860. The trenches could be filled withmaterial that will exhibit an intrinsic compressive or tensile stress onthe light emitting junctions. The strain could be induced through adifference in the thermal expansion coefficients of the material in thetrench and the material surrounding the trench. The trenches could befilled with material and that material could then be treated usinganother processing step to induce the material to cause a compressive ortensile stress on the light emitting junctions. Finally, the trenchescould be exposed to a chemical treatment directly that will cause thenative material to exhibit an intrinsic compressive or tensile stress onthe light emitting junctions.

The strain induced in the light emitting junction can be tuned by thegeometry of the trench pattern, the nature of the treatment applied tothe trenches, the type of material deposited in the trenches, and theconditions under which such materialize are deposited. The geometry ofthe trench pattern can vary as to both the general location of thetrenches on a device relative to the light emitting junctions on thatdevice, and can also vary as to the shape of individual trenches.Regarding the shape of individual trenches, the depth of the trenchrelative to other layers can vary as well as the shape of the sidewallsof the trench. For example, the trench could have a curved shape suchthat the volume of material contained in the trench varied inverselywith the depth of the trench. Each of these approaches is discussed inmore detail immediately below. In all of these approaches, the region ofstrain inducing material can comprise a material having a band gap thatis significantly larger than that of the active material of the junctionsuch that the material will serve as a passivant and insulating materialin addition to providing a straining force to the junction.

Trenches 4850 could be filled with material that will contact thesurrounding substrate, or other layers 4820, and exert either anexpansive or implosive force on the surrounding substrate, or otherlayers, which will in turn exert either a compressive or tensile forceon junction structure 300. The filled trenches will then comprise a setstrain inducing region to tune the optical characteristics of the lightemitting junctions in the device. In certain circumstances, the trencheswill not need to be completely filled to induce strain in the lightemitting junction. The location of the trenches may be configured toinduce either tri-axial, biaxial or uniaxial strain on junctionstructure 4825. The force of the strain inducing trenches can beimparted to the device through the deposition of material that changesits volume after it has been deposited. For example, certain depositedmaterials are deposited in temperatures above or below ambient andexpand or contract as they return to ambient temperatures. A materialthat can exhibit an intrinsic compressive or tensile stress is siliconnitride deposited through PECVD. The strain induced by the siliconnitride will depend on the conditions in which the material is depositedand the geometry of the trenches and trench pattern.

Trenches 4850 could be filled with a material that will not inducestrain until it is treated with an additional processing step. Inparticular, polysilicon could be deposited into the trenches and thenthermally oxidized to produce silicon dioxide. This oxidation processcan achieve a 2.5 volume expansion to the original material and therebyimpart a large in-plane uniaxial or biaxial stress to junction structure4825. The strain induced in the junction could be controlled through theselection of alternative materials or through a modification of thelength and concentration of the oxidation. In addition, differentprocessing steps besides oxidation could be conducted to alter thecharacteristic of the deposited material. Oxidation, or some otherchemical treatment, could also be conducted without filling thetrenches, in which case the expansion or contraction of the material inwhich the trenches were formed would be relied upon to generate a regionof strain inducing material. The top surface of the material in whichthe trenches were formed could be covered by a mask during this step toprevent oxidation of the entire surface of the material.

A specific example of approaches in which the region of strain inducingmaterial is formed using deposition of a material with an intrinsicstrain can be described with reference to FIG. 49A. The figureillustrates a cross section of a light emitting junction device stressorregion at three stages of fabrication 4900, 4901 and 4902. In stage 4900a trench has been etched through a sacrificial mask layer 4907, intoactive layer 4906, and terminates on a barrier layer 4905. The barrierlayer 4905 separates substrate 4904 from active layer 4906. The activelayer 4906 can include several sub-layers used to form the lightemitting semiconductor junction. In stage 4901, a layer of material 4908has been deposited globally across a wafer to fill the trenches. Asdescribed above, this material can provide an intrinsic strain that iscontrolled by the chemistry of the material, the conditions in which thematerial is deposited, and the geometry of the trench. In theillustrated example, material 4908 is silicon nitride deposited viaPECVD, and it will exert a compressive force on a junction that extendsperpendicular to the illustrated cross section. In stage 4902, the layerof material 4908 has been planarizied. The planarization step can beconducted via numerous processes such as a chemical mechanical polish ora chemical etch. The remaining material 4908 will induce strain inactive layer 4906 to thereby tune the optical characteristics of thelight generated by junctions formed in the active layer.

A specific example of approaches in which the region of strain inducingmaterial is formed using a chemical treatment of a trench in the devicecan be described with reference to FIG. 49B. The figure illustrates across section of a light emitting junction device stressor region 4910.In cross section 4910, the trench that was formed in stage 4900 has beenfilled with material 4911. However, it should be noted that additionalmaterial does not need to have been deposited, as the native materialthat surrounds the trench could be directly treated to produce thestrain inducing region. Also, the material 4911 does not need to fillthe trench, as it may only fill a bottom portion of the trench.Furthermore, the point at which the trench is filled can affect theprofile of the strain because most chemical processes will affect theexposed surface of material 4911 most vigorously. As such, the trenchcould be filled half way before being treated to target strain at theactive region that is at the same depth as half the depth of the trench.Material 4911, or the native walls of the trench, may undergo a phase orcomposition change under the influence of the chemical treatment toinduce a strain in junctions of the device. For example, the depositedmaterial could be polysilicon and the chemical treatment could be theintroduction either a dry oxidizer or a wet oxidizer such as steam.Alternatively, the deposited material could be a nitrogen based compoundthat oxidizes through the generation of ammonia. In either case, oxygenatoms 4912 will be introduced to the material 4911 to induce strain inthe surrounding material as illustrated by the lines extending frommaterial 4911 in active layer 4906 and barrier layer 4905. The depositedmaterial could indeed by any kind of semiconductor material that hasbeen oxidized. The oxidation process could be via the lateral oxidationof silicon process (LOCOS) or oxidation of a metal nitride M-N compoundinto an oxide M₂O₃. Likewise, the active layer could be directlyoxidized such that in situations where the active layer comprised metalalloys, the region of straining material could be a metal oxide of thematerial used to form the light emitting junction.

If the chemical treatment discussed immediately above is oxidation,certain benefits can be realized. Since oxidized materials tend to havea high band gap, the strain inducing region can also act as an insulantand passivant. As can be seen in FIG. 49B, if the material were to bedeposited to cover the entire exposed surface of the active layer 4906,or covered the entire exposed surface after being oxidized, the straininducing region could act in tandem with mask layer to provide a fullyencapsulating passivant for the active layer.

The devices described with reference to FIGS. 48A and 49A, similar tothe passivation layer approach, provides certain benefits in that thetrench pattern and trench modification processing can be appliedglobally to a semiconductor wafer. Furthermore, the trenches can beformed after the junction structure 4825 has been finalized such thatthe effect of the strain on the device can be measured. In addition, thetrench approach can induce different kinds of strain such as tri-axial,biaxial and uniaxial strain based on the pattern selected for the trenchholes which provides a refined level of control for tuning the behaviorof the junction. In addition, the trench pattern could be modifiedacross the surface of the wafer such that a single wafer could be usedto test the effect of various different strain levels on a particularjunction material. For example, one device on the wafer could have notrenches and the number of trenches per device could be ramped acrossdifferent devices on the same wafer. This approach would providesignificant benefits because wafer-to-wafer process variations would nothave an effect on the collected measurements thereby isolating theeffect of the pattern on the junctions.

Another strain tuned light emitting semiconductor devices can bedescribed with reference to the wafer cross section 5000 and junctioncross section 5010 in FIGS. 50A and 50B. Wafer cross section 5000illustrates a light emitting semiconductor junction 5003 formed on asubstrate 5001 and encapsulated by region of strain inducing material5002 acting as a wrap-around stressor. Cross section 5010 illustrates atwo dimensional cross section of the wafer cross section 5000 along line5004. FIG. 50B illustrates the light emitting junction 5003 using thereference numerals from FIG. 23 to illustrate how a vertical p-i-njunction with an optional buffer layer could be strained under thisapproach. However, as mentioned above, a light emitting junction withany number of structures could be strained in a similar manner. As shownin cross section 5010, the region of set strain material fullyencompasses the junction in two directions along the illustrated crosssection and is in direct contact with the active layer of the junction.The region of set strain material 5002 also fully encompasses thejunction along a third direction along illustrated cross section (i.e.,the top of the junction is fully covered by the strain inducing regionin this cross section). Therefore, in combination with the substrate, orthe optional buffer layer, the strain inducing region fully encompassesthe junction in every direction along the illustrated cross section. Inan alternative approach, the strain inducing region can leave the topside of the junction exposed to provide space for an electrical contact.

Regions of strain inducing material 5110 and 5155 shown in FIG. 51 areformed using similar methods. In either case, the junction layers 5120and 5165 can be etched to form mesas that will each serve as independentjunctions. A stressed film, or films, can then be deposited over thewafer to produce in-plane stress on the junctions. As an example, atensile stressed film could be deposited globally across the wafer toproduce strain in the junctions. As a specific example, silicon nitrideor tungsten could be deposited across the entire wafer via evaporation.However, multiple films can be deposited using any kind of depositionprocess. The films could also be deposited in sequence such thatdifferent types of strain were introduced at different strata of themesas. As a result, the p-type layer of a p-i-n junction could bestrained using a different strain than that used to strain the n-typelayer of the junction.

A specific example of approaches in which the region of strain inducingmaterial is formed using the backside processing can be described withreference to FIG. 52. The figure illustrates a cross section of a lightemitting junction device at four stages of fabrication 5200-03. Stage5200 illustrates a light emitting junction having active layer 5205,optional barrier layer 5206, and substrate layer 5207. Stage 5201illustrates the device after a stabilizing layer 5208 has been added tothe device and a portion of the substrate has been removed to exposebarrier layer 5206. In an alternative approach, the excavated substratelayer terminates in substrate layer 5207 such that the substrate is onlypartially removed. The stabilizing layer 5208 can be a handle wafer ormechanical chuck used to hold the device wafer in place during back sideprocessing.

In stage 5202, a strain inducing layer of material 5209 has beendeposited on the back side of the device. This material could bedeposited in a targeted manner or it could be globally deposited andthen removed using a planarization step. The strain inducing material5209 could also be a via deposited on the back side of the device. Thevia could be tungsten, copper, or any other conductive material. The viacould also be a portion of a solder bump deposited on the back side ofthe wafer. The strain inducing material could induce strain by having adifferent thermal coefficient of expansion that other layers in thewafer, by having an intrinsic strain due to the characteristics of thebond with the wafer, or by its weight pulling down on the wafer. Straincould also be induced without the deposition of a straining inducingmaterial. For example, active layer 5205 could be a thin membrane thatwould strain under its own weight once the substrate below it and thesupport layer above it were removed. In a combination of theseapproaches, the strain inducing material 5209 could enhance thedeformation of the membrane by increasing the weight that the membraneneeded to support.

In stage 5203, the support layer 5208 has been removed from the wafer.This step could involve reversing a temporary bond between a handlewafer and the device wafer, releasing an electrostatic bond between achuck and the device wafer, releasing an air pressure seal between avacuum chuck and the device wafer, or gradually thinning the supportlayer through a chemical mechanical polish or etching step until adesired level of deformation has been achieved. In this approach, thestrain could be monitored in situ as the support layer was thinned, andthe thinning process could be terminated when the desired strain profilewas achieved. As illustrated, active layer 5205 has warped under theinfluence of its own weight, the strain induced by region of material5209, or a combination of both of those influences. The deformationcould occur in tandem with the removal or thinning of support layer 5208or it could subsequently occur as part of an additional temperaturetreatment or chemical processing step applied to the wafer after supportlayer 5208 was removed or thinned.

A specific example of approaches in which the region of strain inducingmaterial is formed using ion-implantation can be described withreference to FIG. 53. The figure illustrates a cross section of a lightemitting junction device at three stages of fabrication 5300-02. Stage5300 illustrates a light emitting junction device having active layer5305, optional barrier layer 5304, and substrate layer 5303. In thedevice shown in stage 5300, the actual light emitting junction may ormay not have been formed in the device. This is because the straininducing region in this approach can be formed using the same processingapproach used to form the different layers required for the activedevices. A further benefit of this approach is that in certain cases thestrain inducing region and the actual junctions can be formed in eitherorder based on whichever is convenient.

Stage 5301 shows the wafer after a mask 5306 has been placed over thewafer that covers the wafer except for in region 5307. Also illustratedis an ion bombardment 5308 which implants ions to form region 5307. Thision bombardment can introduce impurity ions into anon-intentionally-doped film of active material which could be a III-Nmaterial. The ion bombardment can form the junction structures such asp-type and n-type regions. However, the ion bombardment can also formstrain inducing regions.

In stage 5302, a different mask 5309 has been placed over the wafer.Mask 5309 exposes region 5310. After a second ion-bombardment 5311,region 5310 will have a different characteristic than the rest of activelayer 5305. Again, this region can be part of the junctions, or it canbe a strain inducing region. Various ion bombardments such as 5308 and5311 can be applied to create different regions. By adjusting thechemical composition of the bombardment and the energy of thebombardment, a multitude of potential strain patterns can be induced inthe junctions formed in the device. For example, an oxygen or hydrogenimplant can induce large changes in the material elastic parameters ofactive layer 5305 to create a compressive or tensile force on theportions of active layer 5305 surrounding region 5307 or region 5310.Furthermore, the implanted species can be further activated by a thermalor chemical step such as a thermal anneal and recrystallization toconvert the material such that it induces a different kind of strain.For example, AlGaN can be converted into AlGaON or AlGaO_(x). As aspecific example, an oxygen ion implant of 50 keV can be used to inducestructural changes in the III-N regions. Multiple energy implants canalso be used to control the depth profile of the stressors. For example,in the case of a vertical p-i-n device with an n-type region towards thesurface of the wafer, a low energy implant can be used to strain then-type region alone while leaving the rest of the device unstrained. Asillustrated, implant region 5307 is deeper than region 5310 such thatany strain induced via region 5307 will have a more direct effect on thedeeper regions of layer 5305. Also, although regions 5307 and 5310 aredrawn as if they are homogenous from top to bottom, if the properimplant species and energy are applied, the implant can be conducted totarget certain depths while leaving shallower depths unchanged.

Methods for tuning a light emitting semiconductor junction that are inkeeping with the disclosures above can be described with reference toFIGS. 54-56. These methods allow for the production of light emittingsemiconductor junctions that are tuned to produce a desired emissivitywavelength and polarization that stays within a desired acceptancewindow over a predetermined range of operating conditions. The methodsinvolve the fabrication of a semiconductor junction and the applicationof strain to that junction. In specific instances, the methods includemeasuring the polarization and wavelength of the light generated by thejunctions before applying strain. The methods can be applied to selectthe materials and degree of strain necessary to fabricate the junctions,calibrate the production line used to fabricate the junctions, andassure uniformity across various runs of the production line. Inspecific instances, the methods include iteratively measuring thepolarization and wavelength of the light generated by the junctions andapplying strain to the junctions based on those measurements.

FIG. 54 illustrates a process 5400 for fabricating a DUV-LED with adesired polarization and emission wavelength. As with tuning graph ofFIG. 20, process 5400 is directed to DUV-LEDs because they provide anillustrative application of the processes disclosed in thisspecification, not because the disclosure is limited to DUV-LEDs.Process 5400 begins with step 5401 in which a homojunction is formed togenerate light for the DUV-LED. The homojunction can be formed using thedeposition of various materials and the doping of these materialsthrough chemical vapor deposition, sputtering processes, or ionimplantation. The homojunction can be formed on a substrate of glass,sapphire, native compound semiconductor, silicon or some other kind ofmaterial that differs from the composition of the homojunction. However,the homojunction can also be formed on a bulk substrate that matches thecomposition of the homojunction prior to the introduction of dopants. Asmentioned previously, this can provide certain cost benefits andincrease the flexibility of the fabrication procedure as a greaternumber of fabrication steps are amenable to bulk wafer processing. Thehomojunction can be formed using any of the materials described above.

Process 5400 continues with either step 5402 or 5403. In step 5402, apolarization of the light generated by the homojunction is measured. Instep 5403, a wavelength of the light generated by the homojunction ismeasured. The steps are drawn in parallel and with arrows connecting thetwo steps because they can generally be conducted in any order. Inaddition, it is possible for the two steps to be conducted in parallelusing the same device. However, steps 5402 and 5403 must follow step5401 because it is not possible to measure the light generated by thehomojunction before the device is formed. Step 5401 could include theformation of electrical contacts to the homojunction in order to applyan electromagnetic force used by the device to generate light. Theelectrical contacts could be the permanent contacts that will be used bythe device when it is in finished form. However, the electrical contactscould also be temporary or unfinished contacts that are meant to be usedin combination with a probe device that is part of the fabrication lineused to produce the device.

Process 5400 continues with step 5404 in which a biaxial or uniaxialstrain is induced in the device. The induced strain can be calibratedand applied based on the measurements taken in steps 5402 and 5403. Thestrain can be induced by any of the procedures mentioned above. Inparticular, the strain can be induced via the deposition of apassivation layer across the top surface of the light emitting diodethat applies a biaxial strain to the junction. The passivation layercould be directly in contact with the material that comprises thejunction or there could be various layers in between the junction andthe passivation layer. In the case of a DUV-LED, the strain willgenerally be a biaxial compressive strain intended to switch thepolarization of the emitted light to transverse electric.

After the strain is introduced, process 5400 will continue with step5405 in which the device is packaged. The packaging step could includethe introduction of solder bumps or metal posts in contact with theelectrodes of the DUV-LED and the addition of optical components fordirecting the generated light. In situations where multiple LEDs werebeing fabricated on a wafer, packaging would be preceded by singulatingthe LEDs from the wafer. This singulation could be conducted on sets ofLEDs if keeping multiple LEDs on a single wafer was desired such as inapplications where multiple LEDs were being integrated in a single-chipsystem.

FIG. 55 illustrates a process 5500 for calibrating a fabrication linefor producing devices with light emitting semiconductor junctions.Process 5500 allows a designer to calibrate a production line to producea light emitting semiconductor junction that produces light with adesired emission wavelength and optical polarization. In step 5501, amaterial for the junction will be selected. The material will beselected to generally meet the target emission wavelength range andspecific device requirements of the desired device. Specific devicerequirements include an acceptable lattice match to the buffer layer ofthe device as well as a high activated doping density for the p-type andn-type regions. The optical polarization of the light generated by thematerial can also be taken into account in step 5501 such that if an LEDis required, materials that produce light with transverse electricpolarization can be selected. Returning to the example of a DUV-LED,step 5501 could comprise the selection of AlGaN with the primarycomposition of aluminum exceeding 0.115. The material could be grown asa wurzite crystal on a c-axis oriented buffer of AlN and a sapphiresubstrate. In step 5502, a junction is formed using the selectedmaterial.

In step 5503, a measurement is obtained for the polarization andwavelength of the light generated from the junction. If the measurementindicates that the polarization and wavelength of the generated lightmeet the desired criteria for the device, process 5500 will terminateand the current strain profile and material selection will be saved tocalibrate the manufacturing line. If the measurement indicates that thepolarization and wavelength are not acceptable, strain will be inducedin the device in step 5504 to alter the polarization and wavelength ofthe light as needed. Steps 5503 and 5504 will be repeated iteratively totune both the wavelength and polarization of the emitted light. If theprocess for inducing strain can be iteratively conducted on the samedevice, the process can iteratively cycle directly through steps 5503and 5504. For example, a MEMS could be built into the device or atesting structure could be in contact with the device such that thestrain induced in the junction could be tuned in either direction whilemeasurements were being conducted on that single device. As anotherexample, certain procedures may allow the strain to be tuned in a singledirection in-between measurements—the application of successively moreand more trenches in the excavated trenches approach described withreference to FIG. 48 is an example of this type of tuning mechanism.However, if the process for inducing strain cannot be reliablyoverridden and controlled by a subsequent iteration of step 5504, theprocess may have to cycle back from step 5504 to step 5502 beforeanother measurement could be taken. A subsequent iteration of step 5502could include the production of another junction using the same materialselected in step 5501.

The measurement obtained in step 5503 can be obtained in situ while thedevice is in one of the tools used to form the junction in step 5502 orwhile the device is in the tool used to induce strain in step 5504.However, the measurement could also be obtained after removing thedevice from the tool and placing it on a separate station. The stationcould be one that is specifically designed to conduct the measurementefficiently. Since the calibration of the manufacturing line will notneed to be conducted every time a new batch of wafers is processed, theadditional time needed to remove the device from the assembly line andconduct measurements on the device will not significantly impact thecost of production for the device. One benefit of conducting the tuningof steps 5502-04 in the manufacturing line is that fabricationfacilities generally have stations where the temperature of a wafer anddevice can be monitored and controlled with high accuracy. Therefore,determining if a certain level of induced strain has resulted in adevice that will perform within the desired acceptance window across thedesired range of operating temperatures is generally easier if donewhile the device is in the fabrication facility. Also, some of theprocessing steps associated with steps 5504 and 5502 may be conducted inenvironments where the temperature of the wafer is already controlled aspart of the processing step or is already monitored for quality controlpurposes. Therefore, the device may already be in a conduciveenvironment for obtaining the required measurements for step 5503.

Optional step 5505 will be accessed if a maximum level of strain isinduced, or will be induced in the next instance of step 5504. Asmentioned previously, devices cannot be strained to any desired degreewithout running the risk of degraded performance of the device ortriggering a catastrophic failure of the device's crystal lattice. Themaximum strain can be set based on the qualities of the materialselected in 5501 and measurements of how similar devices have performedover the long term when subjected to certain degrees of strain. If themaximum strain for the device is exceeded before step 5503 obtains asufficient measurement for the device's performance, the method willproceed to step 5501 in which a different material is selected. Thematerial can be reselected based on information obtained from prioriterations of step 5503. For example, if the maximum compressive strainwas exceeded in an Aluminum-based nitride alloy DUV LED without reachingthe desired polarization, then the material selected in step 5501 wouldhave a lower aluminum content than the material that was selected instep 5501 or a previous iteration of step 5505.

FIG. 56 illustrates a process 5600 for fabricating a light emittingsemiconductor junction. Process 5600 begins with step 5601 in which aset of semiconductor junctions are formed on a lot of semiconductorwafers using a semiconductor material. The semiconductor material couldbe any of the materials mentioned above, and, in particular, couldcomprise a three-five material such as a group three nitride alloy. Step5601 can include various processing steps—some of which may involvebatch processing the lot of wafers while others are implemented onewafer at a time. Process 5600 continues with step 5602 in which thelight generated by one of the light emitting junctions is measured. Boththe polarization and the wavelength of the light can be measured in thisstep. The procedure for measuring can match any of those describedabove. In particular, the measurement could be conducted in situ. Also,step 5601 and 5602 may be interspersed such that a first junction isproduced and the light from that junction is measured before anotherjunction in the set is produced.

Process 5600 will then continue with steps 5603 and 5604 in which awafer is strained using a wafer process and then the generated light isremeasured. The wafer process could be the deposition of a straininducing passivation layer, the introduction of strain inducingtrenches, or any of the other methods described above. The process bywhich the light is measured could match the process applied in step 5602or a different measurement process could be applied. For example, thefirst measurement could be conducted in situ and the second measurementcould be conducted at a specialized off-line measurement device. Thetemperature and other operating conditions for both measurements wouldbeneficially be as close to identical as possible in order for the twomeasurements to isolate the effect of the wafer processing step on thegenerated light. The isolated information would then allow themanufacturing system to adjust the strain applied to another wafer inthe lot of wafers in step 5605. For example, if the light measured instep 5604 indicated that the strain needed to be increased, the waferprocessing in step 5605 could be conducted in a way that would induce agreater degree of strain in the device. The manufacturing system couldsave the measurements taken in previous iterations of steps 5602 and5604 to determine what degree of adjustment needed to be made prior toinducing strain in step 5605. This procedure could be conductedperiodically to align the fabrication facility back in line with adesired tolerance.

Process 5600 could also include step 5606 in which the light from thedevice on the second wafer is measured prior to inducing strain in thesecond wafer. This information could then be combined with informationfrom step 5603, step 5604, and the stored information describing prioriterations of those steps to select a proper degree of strain to beapplied in step 5605. Although this process will likely provide agreater degree of confidence and precision in the performance of thedevices on the second wafer, the cost of the additional step should beweighed against the fact that the first and second wafers are part ofthe same lot. Depending on the prior processing steps for the lot thetwo wafers could have substantially the same characteristics such thatstep 5606 serves as an unnecessary repetition of step 5602.

The methods described with reference to both FIG. 55 and FIG. 56 couldbe conducted using a strain inducing wafer processing step that exhibitsa different level of strain across the wafer. The example of varyingnumbers of excavation trenches and patterns for those trenches acrossthe different devices in the wafer is an example of such a varyingglobal wafer process. As a result, a single wafer could be used toobtain several different data points regarding the amount of straininduced in a device and the resulting characteristic of the lightgenerated by the device. When used in combination with process 5500, theefficiency of the original calibration of the manufacturing line couldbe greatly enhanced. When used in combination with process 5600, thevarying global processing step could be applied periodically to acalibration wafer that would be used to adjust the processing step inthe future. The varying global processing step would not be applied toregular production wafers because such an approach would run contrary tothe desire to produce devices having matching and predictablecharacteristics. In either situation, the use of a varying globalprocessing step would allow the manufacturing line to obtain a greaterdegree of information than would be provided by a single waferundergoing a uniform global processing step which would provide clearbenefits to the initial calibration and adjustment of the manufacturingline.

While the specification has been described in detail with respect tospecific embodiments of the invention, it will be appreciated that thoseskilled in the art, upon attaining an understanding of the foregoing,may readily conceive of alterations to, variations of, and equivalentsto these embodiments. These and other modifications and variations tothe present invention may be practiced by those skilled in the art,without departing from the spirit and scope of the present invention,which is more particularly set forth in the appended claims.

What is claimed is:
 1. An ultraviolet light emitting diode, comprising:an ultraviolet light emitting junction; a first electrical contactcoupled to a first side of the junction; a second electrical contactcoupled to a second side of the junction; and a region of set strainingmaterial that exerts a strain on the junction and alters both: (i) anoptical polarization, and (ii) an emission wavelength of the junction;wherein the region of set straining material lies outside a current paththat includes the first electrical contact and the second electricalcontact; and the light emitting semiconductor junction device comprisesa three-five alloy.
 2. The ultraviolet light emitting diode of claim 1,wherein: the junction comprises an epitaxy material; the region of setstraining material comprises a non-epitaxy material; and the opticalpolarization of the junction, as altered by the strain on the junction,is linear.
 3. The ultraviolet light emitting diode of claim 1, wherein:the three-five alloy comprises a ternary group-three nitride alloy; andsaid ternary group-three nitride alloy comprises gallium.
 4. Theultraviolet light emitting diode of claim 1, wherein: a structure of thejunction is selected from a group consisting of: a lateral p-i-nstructure; a superlattice structure; and a multiple quantum wellstructure.
 5. The ultraviolet light emitting diode of claim 1, wherein:the region of set straining material comprises a plurality of externalstressors.
 6. The ultraviolet light emitting diode of claim 5, wherein:the plurality of external stressors all contribute a uniaxial componentto the strain on the junction.
 7. The ultraviolet light emitting diodeof claim 5, wherein: at least one of the plurality of external stressorscontributes a biaxial component to the strain on the junction.
 8. Theultraviolet light emitting diode of claim 5, wherein: at least one ofthe plurality of external stressors contributes a triaxial component tothe strain on the junction.
 9. The ultraviolet light emitting diode ofclaim 1, further comprising: a trench that penetrates a layer ofmaterial used to form the junction; wherein the region of set strainingmaterial includes a bottom portion of the trench.
 10. The ultravioletlight emitting diode of claim 9, wherein: the region of set strainingmaterial comprises an oxidized semiconductor material.
 11. Theultraviolet light emitting diode of claim 1, wherein said region of setstraining material: fully encompasses the junction in two directionsalong a cross section of the junction; and is in direct contact with anactive layer of the junction.
 12. The ultraviolet light emitting diodeof claim 11, further comprising: an n-type layer of the junction, ap-type layer of the junction, and an i-type layer of the junction;wherein the n-type layer, the p-type layer, and the region of setstraining material fully encompass the i-type layer; and wherein theregion of set straining material forms an annular cylinder.
 13. Theultraviolet light emitting diode of claim 12, wherein: the region of setstraining material surrounds a mesa; the mesa comprises the junction;and the region of set straining material leaves a top side of the mesapartially exposed to provide the first electrical contact for thejunction.
 14. The ultraviolet light emitting diode of claim 1, furthercomprising: a substrate layer supporting the junction; and a bufferlayer having a first surface in contact with a top side of thesubstrate; wherein the region of set straining material is also incontact with the first surface of the buffer layer.
 15. The ultravioletlight emitting diode of claim 14, wherein: the region of set strainingmaterial is in an excavated portion of the substrate.
 16. A lightemitting semiconductor junction device, comprising: a light emittingjunction; a first electrical contact coupled to a first side of thejunction; a second electrical contact coupled to a second side of thejunction; and a region of set straining material that exerts a strain onthe junction and alters both: (i) an optical polarization, and (ii)emission wavelength of the junction; wherein: the first electricalcontact and the second electrical contact provide a current path to thelight emitting junction independent of the region of set strainingmaterial; the region of set straining material covers a third side and afourth side of the light emitting junction along a cross section of thelight emitting junction; and the light emitting semiconductor junctiondevice comprises a three-five alloy.
 17. The light emittingsemiconductor junction device of claim 16, wherein: the light emittingsemiconductor junction device is a deep ultraviolet diode; the lightemitting junction comprises a ternary group-three nitride alloy; and theternary group-three nitride alloy comprises gallium.
 18. The lightemitting semiconductor junction device of claim 17, wherein: the lightemitting junction comprises aluminum.
 19. The light emittingsemiconductor junction device of claim 16, wherein: the light emittingsemiconductor junction device is a vertical p-i-n deep ultravioletdiode; and the region of set straining material forms an annularcylinder.
 20. The light emitting semiconductor junction device of claim16, wherein: said light emitting semiconductor junction device is alateral p-i-n ultraviolet diode; and the region of set strainingmaterial covers a fifth side of the light emitting junction along thecross section.
 21. A light emitting semiconductor junction device,comprising: a light emitting junction; a first electrical contactcoupled to a first side of the junction; a second electrical contactcoupled to a second side of the junction; and a region of set strainingmaterial that exerts a strain on the junction and alters both: (i) anoptical polarization, and (ii) emission wavelength of the junction;wherein: the region of set straining material is not on a current pathbetween said first electrical contact and said second electricalcontact; said region of set straining material covers a third side and afourth side of the light emitting junction along a cross section of thelight emitting junction; the light emitting semiconductor junctiondevice comprises a three-five alloy.
 22. The light emittingsemiconductor junction device of claim 21, further comprising: a trenchthat penetrates a layer of material used to form the junction; and anoxidized polysilicon material formed in the trench; wherein the regionof set straining material comprises the oxidized polysilicon material.23. The light emitting semiconductor junction device of claim 21,further comprising: a trench that penetrates a layer of material used toform the junction; wherein the region of set straining materialcomprises a metal-oxide of the layer of material.
 24. The light emittingsemiconductor junction device of claim 21, wherein: the light emittingsemiconductor junction device is an ultraviolet diode; the lightemitting semiconductor junction device comprises a ternary group-threenitride alloy; and the ternary group-three nitride alloy comprisesgallium and aluminum.
 25. The light emitting semiconductor junctiondevice of claim 24, further comprising: a trench that penetrates a layerof the ternary group-three nitride alloy; wherein the region of setstraining material comprises a metal-oxide of the ternary group-threenitride alloy.